Functions & Closures: From JS to PHP

Overview

Functions in PHP and TypeScript/JavaScript share many concepts, but PHP's approach to closures and variable scope has important differences. This chapter bridges your JavaScript knowledge to PHP's function system.

Learning Objectives

By the end of this chapter, you'll be able to:

• ✅ Write functions with proper type hints in PHP
• ✅ Understand PHP's closure scope and variable capture
• ✅ Create higher-order functions
• ✅ Use first-class functions and callbacks
• ✅ Work with callable types
• ✅ Implement generator functions (yield)
• ✅ Apply functional programming patterns

Code Examples

📁 View Code Examples on GitHub

This chapter includes comprehensive function and closure examples:

• Closure variable capture patterns
• Higher-order functions
• Generators and lazy evaluation
• Currying and memoization techniques

Run the examples:

cd code/php-typescript-developers/chapter-03
php closures.php
php generators.php

Function Declarations

TypeScript

// Function declaration
function add(a: number, b: number): number {
  return a + b;
}

// Arrow function
const multiply = (a: number, b: number): number => a * b;

// Optional parameters
function greet(name: string, greeting?: string): string {
  return `${greeting ?? "Hello"}, ${name}!`;
}

// Default parameters
function createUser(name: string, age: number = 18): User {
  return { name, age };
}

// Rest parameters
function sum(...numbers: number[]): number {
  return numbers.reduce((a, b) => a + b, 0);
}

PHP

<?php
declare(strict_types=1);

// Function declaration
function add(int $a, int $b): int {
    return $a + $b;
}

// Arrow function (single expression only)
$multiply = fn(int $a, int $b): int => $a * $b;

// Optional parameters (use nullable type + null default)
function greet(string $name, ?string $greeting = null): string {
    return ($greeting ?? "Hello") . ", {$name}!";
}

// Default parameters
function createUser(string $name, int $age = 18): array {
    return compact('name', 'age');
}

// Variadic parameters (rest parameters)
function sum(int ...$numbers): int {
    return array_sum($numbers);
}

Key Differences:

• PHP uses ... before the parameter name (...$numbers)
• PHP arrow functions use fn keyword
• Optional parameters typically use nullable types with null defaults

Closures and Variable Scope

The Critical Difference: Lexical Scope

TypeScript/JavaScript:

// Variables are automatically captured
const multiplier = 2;

const double = (x: number): number => {
  return x * multiplier; // ✅ Automatically accesses outer scope
};

console.log(double(5)); // 10

PHP:

<?php
$multiplier = 2;

// ❌ Does NOT automatically capture outer variables
$double = function(int $x): int {
    return $x * $multiplier; // ❌ Error: Undefined variable $multiplier
};

// ✅ Must explicitly use 'use' clause
$double = function(int $x) use ($multiplier): int {
    return $x * $multiplier; // ✅ Works
};

echo $double(5); // 10

PHP Arrow Functions (PHP 7.4+):

<?php
$multiplier = 2;

// ✅ Arrow functions automatically capture variables
$double = fn(int $x): int => $x * $multiplier;

echo $double(5); // 10

Closure Variable Capture

TypeScript:

let counter = 0;

const increment = (): void => {
  counter++; // ✅ Mutates outer variable
};

increment();
console.log(counter); // 1

PHP (Reference Capture):

<?php
$counter = 0;

// By value (default) - does NOT mutate outer variable
$increment = function() use ($counter): void {
    $counter++; // ❌ Only modifies local copy
};

$increment();
echo $counter; // 0 (unchanged)

// By reference - DOES mutate outer variable
$increment = function() use (&$counter): void {
    $counter++; // ✅ Modifies outer variable
};

$increment();
echo $counter; // 1 (changed)

PHP Arrow Functions:

<?php
$counter = 0;

// Arrow functions capture by value (cannot modify outer variables)
$increment = fn() => $counter++; // ❌ Does not modify outer $counter

// For mutation, use traditional closure with reference
$increment = function() use (&$counter): void {
    $counter++;
};

Higher-Order Functions

Functions Returning Functions

TypeScript:

const makeMultiplier = (factor: number) => {
  return (value: number): number => value * factor;
};

const double = makeMultiplier(2);
const triple = makeMultiplier(3);

console.log(double(5)); // 10
console.log(triple(5)); // 15

PHP:

<?php
declare(strict_types=1);

function makeMultiplier(int $factor): callable {
    return fn(int $value): int => $value * $factor;
}

$double = makeMultiplier(2);
$triple = makeMultiplier(3);

echo $double(5) . PHP_EOL; // 10
echo $triple(5) . PHP_EOL; // 15

Functions as Arguments (Callbacks)

TypeScript:

const numbers = [1, 2, 3, 4, 5];

// map, filter, reduce
const doubled = numbers.map(x => x * 2);
const evens = numbers.filter(x => x % 2 === 0);
const sum = numbers.reduce((acc, val) => acc + val, 0);

PHP:

<?php
$numbers = [1, 2, 3, 4, 5];

// array_map, array_filter, array_reduce
$doubled = array_map(fn($x) => $x * 2, $numbers);
$evens = array_filter($numbers, fn($x) => $x % 2 === 0);
$sum = array_reduce($numbers, fn($acc, $val) => $acc + $val, 0);

Custom Higher-Order Function:

TypeScript:

function applyOperation<T>(
  items: T[],
  operation: (item: T) => T
): T[] {
  return items.map(operation);
}

const numbers = [1, 2, 3];
const doubled = applyOperation(numbers, x => x * 2);

PHP:

<?php
declare(strict_types=1);

/**
 * @template T
 * @param array<T> $items
 * @param callable(T): T $operation
 * @return array<T>
 */
function applyOperation(array $items, callable $operation): array {
    return array_map($operation, $items);
}

$numbers = [1, 2, 3];
$doubled = applyOperation($numbers, fn($x) => $x * 2);

Callable Type Hints

TypeScript Function Types

type MathOperation = (a: number, b: number) => number;

const add: MathOperation = (a, b) => a + b;
const multiply: MathOperation = (a, b) => a * b;

function calculate(
  a: number,
  b: number,
  operation: MathOperation
): number {
  return operation(a, b);
}

PHP Callable Types

<?php
declare(strict_types=1);

// Simple callable type hint
function calculate(int $a, int $b, callable $operation): int {
    return $operation($a, $b);
}

// Using PHPStan/Psalm docblock for precise typing
/**
 * @param callable(int, int): int $operation
 */
function calculateWithDoc(int $a, int $b, callable $operation): int {
    return $operation($a, $b);
}

// Usage
$add = fn($a, $b) => $a + $b;
$multiply = fn($a, $b) => $a * $b;

echo calculate(5, 3, $add); // 8
echo calculate(5, 3, $multiply); // 15

PHP Callable Formats:

<?php
// 1. Anonymous function
$callback = function() { return "hello"; };

// 2. Arrow function
$callback = fn() => "hello";

// 3. Function name as string
$callback = 'strlen';

// 4. Static method (array format)
$callback = [MyClass::class, 'staticMethod'];

// 5. Object method (array format)
$callback = [$object, 'method'];

// 6. First-class callable (PHP 8.1+)
$callback = strlen(...);

First-Class Callable Syntax (PHP 8.1+)

TypeScript

// Functions are first-class citizens
const fn = Math.max;
console.log(fn(1, 2, 3)); // 3

PHP (8.1+)

<?php
// Old way (string or array)
$fn = 'max';
echo $fn(1, 2, 3); // 3

// First-class callable syntax (8.1+)
$fn = max(...);
echo $fn(1, 2, 3); // 3

// Object methods
$fn = $object->method(...);
$result = $fn($arg);

// Static methods
$fn = MyClass::staticMethod(...);
$result = $fn($arg);

Generators (yield)

TypeScript Generators

function* generateNumbers(max: number): Generator<number> {
  for (let i = 0; i < max; i++) {
    yield i;
  }
}

for (const num of generateNumbers(5)) {
  console.log(num); // 0, 1, 2, 3, 4
}

PHP Generators

<?php
function generateNumbers(int $max): Generator {
    for ($i = 0; $i < $max; $i++) {
        yield $i;
    }
}

foreach (generateNumbers(5) as $num) {
    echo $num . PHP_EOL; // 0, 1, 2, 3, 4
}

Generator with Keys:

<?php
function generateKeyValue(): Generator {
    yield 'a' => 1;
    yield 'b' => 2;
    yield 'c' => 3;
}

foreach (generateKeyValue() as $key => $value) {
    echo "{$key}: {$value}" . PHP_EOL;
}

Practical Example - Lazy Loading:

TypeScript:

function* readLargeFile(filename: string): Generator<string> {
  const lines = fs.readFileSync(filename, 'utf-8').split('\n');
  for (const line of lines) {
    yield line;
  }
}

PHP:

<?php
function readLargeFile(string $filename): Generator {
    $handle = fopen($filename, 'r');
    while (($line = fgets($handle)) !== false) {
        yield trim($line);
    }
    fclose($handle);
}

// Memory-efficient: processes one line at a time
foreach (readLargeFile('large.txt') as $line) {
    // Process line
}

Recursion

TypeScript

const factorial = (n: number): number => {
  if (n <= 1) return 1;
  return n * factorial(n - 1);
};

console.log(factorial(5)); // 120

PHP

<?php
declare(strict_types=1);

function factorial(int $n): int {
    if ($n <= 1) return 1;
    return $n * factorial($n - 1);
}

echo factorial(5); // 120

Tail-Call Optimization (Not in PHP):

PHP does not optimize tail calls, so deep recursion can cause stack overflow. Use iteration or trampolining for deep recursion.

Currying and Partial Application

TypeScript Currying

// Curried function
const add = (a: number) => (b: number) => a + b;

const add5 = add(5);
console.log(add5(10)); // 15

// Generic curry helper
function curry<A, B, C>(fn: (a: A, b: B) => C) {
  return (a: A) => (b: B) => fn(a, b);
}

const multiply = (a: number, b: number) => a * b;
const curriedMultiply = curry(multiply);
const double = curriedMultiply(2);
console.log(double(10)); // 20

PHP Currying

<?php
declare(strict_types=1);

// Curried function
$add = fn(int $a): callable => fn(int $b): int => $a + $b;

$add5 = $add(5);
echo $add5(10); // 15

// Generic curry helper
function curry(callable $fn): callable {
    return function(...$args) use ($fn): callable {
        return function(...$moreArgs) use ($fn, $args): mixed {
            return $fn(...array_merge($args, $moreArgs));
        };
    };
}

$multiply = fn(int $a, int $b): int => $a * $b;
$curriedMultiply = curry($multiply);
$double = $curriedMultiply(2);
echo $double(10); // 20

Partial Application

TypeScript:

const greet = (greeting: string, name: string) => `${greeting}, ${name}!`;

const sayHello = (name: string) => greet("Hello", name);
const sayHi = (name: string) => greet("Hi", name);

console.log(sayHello("Alice")); // "Hello, Alice!"
console.log(sayHi("Bob"));      // "Hi, Bob!"

PHP:

<?php
declare(strict_types=1);

$greet = fn(string $greeting, string $name): string => "{$greeting}, {$name}!";

// Partial application with arrow functions
$sayHello = fn(string $name): string => $greet("Hello", $name);
$sayHi = fn(string $name): string => $greet("Hi", $name);

echo $sayHello("Alice"); // "Hello, Alice!"
echo $sayHi("Bob");      // "Hi, Bob!"

// Or with traditional closures for clarity
$sayHello = function(string $name) use ($greet): string {
    return $greet("Hello", $name);
};

Memoization Pattern

TypeScript Memoization

function memoize<T extends (...args: any[]) => any>(fn: T): T {
  const cache = new Map();

  return ((...args: any[]) => {
    const key = JSON.stringify(args);
    if (cache.has(key)) {
      return cache.get(key);
    }
    const result = fn(...args);
    cache.set(key, result);
    return result;
  }) as T;
}

const expensiveOperation = (n: number): number => {
  console.log(`Computing ${n}...`);
  return n * n;
};

const memoized = memoize(expensiveOperation);
console.log(memoized(5)); // "Computing 5..." -> 25
console.log(memoized(5)); // 25 (cached, no log)

PHP Memoization

<?php
declare(strict_types=1);

function memoize(callable $fn): callable {
    $cache = [];

    return function(...$args) use ($fn, &$cache): mixed {
        $key = json_encode($args);

        if (!isset($cache[$key])) {
            $cache[$key] = $fn(...$args);
        }

        return $cache[$key];
    };
}

$expensiveOperation = function(int $n): int {
    echo "Computing {$n}...\n";
    return $n * $n;
};

$memoized = memoize($expensiveOperation);
echo $memoized(5) . PHP_EOL; // "Computing 5..." -> 25
echo $memoized(5) . PHP_EOL; // 25 (cached, no log)
echo $memoized(10) . PHP_EOL; // "Computing 10..." -> 100

Key Point: PHP requires &$cache to modify the cache array across calls.

Common Closure Pitfalls

Pitfall 1: Forgetting to Capture Variables

<?php
$multiplier = 10;

// ❌ BAD: Closure doesn't capture $multiplier
$double = function(int $x): int {
    return $x * $multiplier; // Error: Undefined variable
};

// ✅ GOOD: Explicit capture
$double = function(int $x) use ($multiplier): int {
    return $x * $multiplier;
};

// ✅ BETTER: Use arrow function for auto-capture
$double = fn(int $x): int => $x * $multiplier;

Pitfall 2: Capturing by Value vs Reference

<?php
$counter = 0;

// ❌ WRONG: Captures by value (doesn't mutate outer)
$increment = function() use ($counter): void {
    $counter++; // Only modifies local copy
};

$increment();
echo $counter; // Still 0

// ✅ CORRECT: Capture by reference
$counter = 0;
$increment = function() use (&$counter): void {
    $counter++; // Modifies outer variable
};

$increment();
echo $counter; // 1

Pitfall 3: Late Binding in Loops

TypeScript:

const callbacks: (() => void)[] = [];

for (let i = 0; i < 3; i++) {
  callbacks.push(() => console.log(i)); // ✅ Each closure captures its own i
}

callbacks.forEach(cb => cb()); // 0, 1, 2

PHP (Problematic):

<?php
$callbacks = [];

// ❌ WRONG: All closures share same $i reference
for ($i = 0; $i < 3; $i++) {
    $callbacks[] = function() use ($i) {
        echo $i . PHP_EOL;
    };
}

foreach ($callbacks as $cb) {
    $cb(); // 2, 2, 2 (all capture final value)
}

// ✅ CORRECT: Capture by value explicitly
$callbacks = [];
for ($i = 0; $i < 3; $i++) {
    $callbacks[] = function() use ($i) { // Captures current value
        echo $i . PHP_EOL;
    };
}

foreach ($callbacks as $cb) {
    $cb(); // 0, 1, 2
}

// ✅ ALTERNATIVE: Use arrow function
$callbacks = [];
for ($i = 0; $i < 3; $i++) {
    $callbacks[] = fn() => print($i . PHP_EOL);
}

Explanation: In the first example, if you use use (&$i) (reference), all closures point to the same variable, which ends at 2. Using use ($i) (value) captures the value at creation time.

Pitfall 4: Arrow Functions Can't Modify Captured Variables

<?php
$count = 0;

// ❌ Arrow functions can't modify captured variables
$increment = fn() => $count++; // Modifies local copy, not outer

$increment();
echo $count; // Still 0

// ✅ Use traditional closure with reference
$increment = function() use (&$count): void {
    $count++;
};

$increment();
echo $count; // 1

Pitfall 5: Closure Serialization

<?php
// ❌ Closures cannot be serialized
$fn = fn($x) => $x * 2;
$serialized = serialize($fn); // Error: Serialization of 'Closure' is not allowed

// ✅ Use named functions or Opis/Closure library
function double(int $x): int {
    return $x * 2;
}

$serialized = serialize('double'); // OK
$fn = unserialize($serialized);
echo $fn(5); // 10

Function Composition

TypeScript Compose

const compose = <T>(...fns: Array<(arg: T) => T>) => {
  return (value: T): T => {
    return fns.reduceRight((acc, fn) => fn(acc), value);
  };
};

const add2 = (x: number) => x + 2;
const multiply3 = (x: number) => x * 3;
const square = (x: number) => x ** 2;

const compute = compose(square, multiply3, add2);
console.log(compute(5)); // square(multiply3(add2(5))) = square(21) = 441

PHP Compose

<?php
declare(strict_types=1);

function compose(callable ...$functions): callable {
    return function(mixed $value) use ($functions): mixed {
        return array_reduce(
            array_reverse($functions),
            fn($carry, $fn) => $fn($carry),
            $value
        );
    };
}

$add2 = fn(int $x): int => $x + 2;
$multiply3 = fn(int $x): int => $x * 3;
$square = fn(int $x): int => $x ** 2;

$compute = compose($square, $multiply3, $add2);
echo $compute(5); // 441

Closure Binding and $this Context

TypeScript this Binding

class Counter {
  private count = 0;

  // Arrow function preserves this
  increment = () => {
    this.count++;
  };

  // Regular method needs binding
  incrementMethod() {
    this.count++;
  }

  getCount() {
    return this.count;
  }
}

const counter = new Counter();
const inc = counter.increment;
inc(); // ✅ Works (arrow function bound to instance)

const incMethod = counter.incrementMethod;
incMethod(); // ❌ Error: this is undefined

PHP Closure Binding

<?php
declare(strict_types=1);

class Counter {
    private int $count = 0;

    public function increment(): void {
        $this->count++;
    }

    public function getCount(): int {
        return $this->count;
    }

    public function getIncrementer(): callable {
        // Closure has access to $this automatically
        return function(): void {
            $this->count++; // ✅ Works
        };
    }

    public function getArrowIncrementer(): callable {
        // Arrow function also has $this access
        return fn() => $this->count++;
    }
}

$counter = new Counter();
$inc = $counter->getIncrementer();
$inc();
echo $counter->getCount(); // 1

// Rebinding closure to different object
$counter2 = new Counter();
$boundInc = $inc->bindTo($counter2);
$boundInc();
echo $counter2->getCount(); // 1

Key Difference: PHP closures defined inside methods automatically have access to $this, unlike JavaScript where you need arrow functions or explicit binding.

Practical Example: Event Emitter

Let's build a simple event emitter in both languages:

TypeScript

class EventEmitter {
  private listeners: Map<string, Array<(...args: any[]) => void>> = new Map();

  on(event: string, callback: (...args: any[]) => void): void {
    if (!this.listeners.has(event)) {
      this.listeners.set(event, []);
    }
    this.listeners.get(event)!.push(callback);
  }

  emit(event: string, ...args: any[]): void {
    const callbacks = this.listeners.get(event);
    if (callbacks) {
      callbacks.forEach(callback => callback(...args));
    }
  }
}

// Usage
const emitter = new EventEmitter();
emitter.on('message', (msg: string) => console.log(`Received: ${msg}`));
emitter.emit('message', 'Hello!'); // "Received: Hello!"

PHP

<?php
declare(strict_types=1);

class EventEmitter {
    /** @var array<string, array<callable>> */
    private array $listeners = [];

    public function on(string $event, callable $callback): void {
        if (!isset($this->listeners[$event])) {
            $this->listeners[$event] = [];
        }
        $this->listeners[$event][] = $callback;
    }

    public function emit(string $event, mixed ...$args): void {
        if (isset($this->listeners[$event])) {
            foreach ($this->listeners[$event] as $callback) {
                $callback(...$args);
            }
        }
    }
}

// Usage
$emitter = new EventEmitter();
$emitter->on('message', fn($msg) => echo "Received: {$msg}" . PHP_EOL);
$emitter->emit('message', 'Hello!'); // "Received: Hello!"

Hands-On Exercise

Task 1: Implement Array Methods

Implement myMap, myFilter, and myReduce functions in PHP:

Goal:

<?php
$numbers = [1, 2, 3, 4, 5];
$doubled = myMap($numbers, fn($x) => $x * 2);
$evens = myFilter($numbers, fn($x) => $x % 2 === 0);
$sum = myReduce($numbers, fn($acc, $val) => $acc + $val, 0);

Solution

<?php
declare(strict_types=1);

/**
 * @template T
 * @template U
 * @param array<T> $array
 * @param callable(T): U $callback
 * @return array<U>
 */
function myMap(array $array, callable $callback): array {
    $result = [];
    foreach ($array as $item) {
        $result[] = $callback($item);
    }
    return $result;
}

/**
 * @template T
 * @param array<T> $array
 * @param callable(T): bool $callback
 * @return array<T>
 */
function myFilter(array $array, callable $callback): array {
    $result = [];
    foreach ($array as $item) {
        if ($callback($item)) {
            $result[] = $item;
        }
    }
    return $result;
}

/**
 * @template T
 * @template U
 * @param array<T> $array
 * @param callable(U, T): U $callback
 * @param U $initial
 * @return U
 */
function myReduce(array $array, callable $callback, mixed $initial): mixed {
    $accumulator = $initial;
    foreach ($array as $item) {
        $accumulator = $callback($accumulator, $item);
    }
    return $accumulator;
}

// Test
$numbers = [1, 2, 3, 4, 5];
$doubled = myMap($numbers, fn($x) => $x * 2);
$evens = myFilter($numbers, fn($x) => $x % 2 === 0);
$sum = myReduce($numbers, fn($acc, $val) => $acc + $val, 0);

print_r($doubled); // [2, 4, 6, 8, 10]
print_r($evens);   // [2, 4]
echo $sum;         // 15

Task 2: Pipeline Function

Create a pipe function that chains multiple operations:

Goal:

<?php
$result = pipe(
    5,
    fn($x) => $x * 2,  // 10
    fn($x) => $x + 3,  // 13
    fn($x) => $x ** 2  // 169
);
echo $result; // 169

Solution

<?php
declare(strict_types=1);

function pipe(mixed $value, callable ...$functions): mixed {
    foreach ($functions as $fn) {
        $value = $fn($value);
    }
    return $value;
}

// Test
$result = pipe(
    5,
    fn($x) => $x * 2,  // 10
    fn($x) => $x + 3,  // 13
    fn($x) => $x ** 2  // 169
);

echo $result; // 169

Alternative with array_reduce:

<?php
function pipe(mixed $value, callable ...$functions): mixed {
    return array_reduce(
        $functions,
        fn($carry, $fn) => $fn($carry),
        $value
    );
}

Task 3: Debounce Function

Implement a simple debounce function:

Solution

<?php
declare(strict_types=1);

function debounce(callable $callback, int $delayMs): callable {
    $lastCall = 0;

    return function(...$args) use ($callback, $delayMs, &$lastCall) {
        $now = (int)(microtime(true) * 1000);

        if ($now - $lastCall >= $delayMs) {
            $lastCall = $now;
            return $callback(...$args);
        }

        return null;
    };
}

// Usage
$logMessage = debounce(
    fn($msg) => echo "Log: {$msg}" . PHP_EOL,
    1000 // 1 second debounce
);

$logMessage("Hello"); // Prints
$logMessage("World"); // Ignored (within 1s)
sleep(2);
$logMessage("Again"); // Prints (after 1s)

Key Takeaways

1. Closures require explicit variable capture with use clause (except arrow functions)
2. Arrow functions (fn) automatically capture variables but only support single expressions
3. Variable capture by reference requires & in use (&$var) for mutation
4. Arrow functions can't modify captured variables—use traditional closures with & for that
5. Callable type accepts functions, closures, method references, and string function names
6. First-class callable syntax (fn(...)) available in PHP 8.1+ for cleaner references
7. Generators work identically to TypeScript/JavaScript for lazy evaluation
8. No tail-call optimization in PHP—use iteration for deep recursion
9. Currying and partial application possible but require manual implementation
10. Memoization requires &$cache reference to persist cache across invocations
11. Loop variable capture behaves differently—use by-value use ($i) to capture current iteration
12. Closures cannot be serialized—use named functions or external libraries
13. Function composition can be implemented with array_reduce and array_reverse
14. PHP closures in methods automatically have access to $this (no need for binding like JS)
15. bindTo() method allows rebinding closures to different object instances

Comparison Table

Feature	TypeScript	PHP
Arrow Function	(x) => x * 2	fn($x) => $x * 2
Multi-line Arrow	✅	❌ (use function)
Auto Capture Vars	✅	✅ (arrow), ❌ (function)
Explicit Capture	N/A	use ($var)
Reference Capture	Default	use (&$var)
Generators	function*	function + yield
Callable Type	(a: T) => U	callable
First-Class Callable	Native	fn(...) (PHP 8.1+)
Tail-Call Optimization	Engine-dependent	❌

Next Steps

Now that you understand functions and closures, let's explore object-oriented programming in PHP.

Next Chapter: 04: OOP: Classes, Interfaces & Generics

Resources

• PHP Closures Documentation
• PHP Arrow Functions RFC
• PHP Generators Documentation
• PHP First-Class Callable Syntax

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