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Control Structures

Chapter 03: Control Structures ​

Overview ​

So far, our scripts have run in a straight line, from top to bottom. But to build interesting applications, our code needs to be able to make decisions and perform repetitive tasks. For example, we might want to show a "Welcome back!" message if a user is logged in, or display a list of products from a database.

This is where control structures come in. They allow us to control the "flow" of our code's execution. In this chapter, you'll learn about the two main types: conditionals (for making decisions) and loops (for repeating actions).

Prerequisites ​

  • PHP 8.4 installed and working (verify with php --version)
  • A text editor or IDE
  • Basic understanding of variables, data types, and arrays from previous chapters
  • Estimated time: ~30 minutes

What You'll Build ​

By the end of this chapter, you'll have:

  • control-structures.php — A playground file showcasing conditionals and loop patterns
  • switch-vs-match.php — Examples comparing switch statements and modern match expressions
  • loop-control.php — Demonstrations of break, continue, and common loop pitfalls
  • A set of array iteration patterns using foreach
  • Practical exercises such as FizzBuzz, multiplication tables, and a number guessing game

Objectives ​

  • Use if, elseif, and else to execute code based on conditions
  • Understand comparison operators (==, ===, !=, !==, >, >=, <, <=)
  • Combine conditions with logical operators (&&, ||, !)
  • Use switch statements and modern match expressions for multiple conditions
  • Apply ternary operator (? :) and null coalescing operator (??) for concise conditionals
  • Use for loops to repeat a task a specific number of times
  • Use while loops to repeat a task as long as a condition is true
  • Use foreach loops to easily iterate over arrays
  • Control loop flow with break and continue statements
  • Troubleshoot common control structure issues

Step 1: Making Decisions with if, elseif, and else (~5 min) ​

Goal ​

Use conditional statements to execute different code paths based on the current state.

Actions ​

Think of an if statement as a decision tree: if a condition is true, run a block of code; otherwise, skip it. You can extend the decision tree with elseif and provide a fallback with else.

  1. Create a file called control-structures.php.
  2. Add the following code:
php
# filename: control-structures.php
<?php

$hour = date('H'); // Gets the current hour in 24-hour format

if ($hour < 12) {
    echo "Good morning!";
} elseif ($hour < 18) {
    echo "Good afternoon!";
} else {
    echo "Good evening!";
}

echo PHP_EOL;
  1. Run the script:
bash
php control-structures.php

Expected Result ​

The greeting changes based on the time of day. You might see:

Good afternoon!

Why It Works ​

  • date('H') returns the current hour as a zero-padded integer string (00–23).
  • The condition $hour < 12 evaluates to true if it’s before noon; otherwise PHP checks the next condition.
  • Only the first matching branch executes, ensuring a single greeting is printed.

Troubleshooting ​

  • Always prints “Good evening!” — Confirm your system clock and timezone (date_default_timezone_set) are set correctly.
  • Syntax error near elseif — Ensure all opening braces { have matching closing braces } and each conditional keyword is lowercase.
  • Output appears on the same line as the prompt — Append PHP_EOL (as shown) to add a newline after the greeting.

Understanding Comparison Operators ​

The condition inside the if parentheses must evaluate to either true or false. We create these conditions using comparison operators.

OperatorNameExampleResult
==Equal$a == $btrue if $a is equal to $b (type coercion)
===Identical$a === $btrue if $a equals $b AND same type
!=Not equal$a != $btrue if $a is not equal to $b
!==Not identical$a !== $btrue if $a is not equal to $b OR different type
<Less than$a < $btrue if $a is less than $b
>Greater than$a > $btrue if $a is greater than $b
<=Less than or equal to$a <= $btrue if $a is less or equal to $b
>=Greater than or equal to$a >= $btrue if $a is greater or equal to $b

Pro Tip: Always prefer the "identical" operator (===) over the "equal" operator (==). The identical operator checks that both the value and the data type are the same, which prevents subtle bugs. For example, 0 == "0" is true, but 0 === "0" is false.

Understanding Logical Operators ​

Often you'll need to combine multiple conditions. That's where logical operators come in.

OperatorNameExampleResult
&&And$a > 5 && $b < 10true if both conditions are true
||Or$a > 5 || $b < 10true if at least one condition is true
!Not!$isLoggedInInverts the boolean value (true becomes false)

Example:

php
<?php

$age = 25;
$hasLicense = true;

if ($age >= 18 && $hasLicense) {
    echo "You can drive!" . PHP_EOL;
} else {
    echo "You cannot drive." . PHP_EOL;
}

Step 2: Repeating Tasks with a for Loop (~3 min) ​

Goal ​

Repeat an action a specific number of times using a for loop.

The for loop consists of three parts inside its parentheses, separated by semicolons:

  1. Initialization — Runs once at the beginning ($i = 1).
  2. Condition — Checked before each iteration. If it’s true, the loop continues ($i <= 10).
  3. Increment — Runs at the end of each iteration ($i++).

Actions ​

  1. Add the following code to control-structures.php (or create a separate file if you prefer):
php
<?php

// This loop will run 10 times.
for ($i = 1; $i <= 10; $i++) {
    echo "This is loop number " . $i . PHP_EOL;
}
  1. Run the script:
bash
php control-structures.php

Expected Result ​

This is loop number 1
This is loop number 2
This is loop number 3
This is loop number 4
This is loop number 5
This is loop number 6
This is loop number 7
This is loop number 8
This is loop number 9
This is loop number 10

Why It Works ​

  • $i starts at 1 and increments by 1 after each iteration.
  • The loop stops once $i becomes 11 because the condition $i <= 10 fails.
  • Concatenation (.) combines the static text with the current value of $i.

Troubleshooting ​

  • Loop never stops — Double-check the condition and increment. If you accidentally wrote $i >= 10, the loop won’t terminate.
  • No output — Ensure the loop body contains an echo statement and that the script runs without syntax errors.
  • Numbers appear on one line — Add PHP_EOL or \n to include line breaks.

Step 3: Repeating Tasks with a while Loop (~3 min) ​

Goal ​

Continue executing a block of code as long as a condition remains true.

Actions ​

  1. Add the following code to your script:
php
<?php

$randomNumber = 0;
$count = 0;

// Keep looping until the number is 5.
while ($randomNumber !== 5) {
    $randomNumber = rand(1, 10); // Generate a random number between 1 and 10
    $count++;
    echo "Attempt #$count: The number is $randomNumber" . PHP_EOL;
}

echo "Success! It took $count attempts to get the number 5." . PHP_EOL;
  1. Run the script:
bash
php control-structures.php

Expected Result ​

The output will vary, but it looks something like:

Attempt #1: The number is 3
Attempt #2: The number is 7
Attempt #3: The number is 5
Success! It took 3 attempts to get the number 5.

Why It Works ​

  • while evaluates the condition before each iteration. If it’s true, the loop executes; otherwise, it stops.
  • The loop continues generating numbers until rand() returns 5.
  • $count tracks how many attempts were needed, providing useful feedback at the end.

Troubleshooting ​

  • Infinite loop — Ensure the loop changes the variables in the condition ($randomNumber). Without updating it, the loop never exits.
  • Same number every time — rand() is pseudo-random but should vary. If it doesn’t, check for mistakes or consider mt_rand().
  • Off-by-one errors — Make sure you’re comparing with the correct condition (!== 5). Using != works too but strict comparison is safer for type checks.

Step 4: Looping Through Arrays with foreach (~4 min) ​

Goal ​

Iterate over arrays cleanly using foreach without managing indexes manually.

Actions ​

  1. Loop through values:
php
<?php

$colors = ['Red', 'Green', 'Blue', 'Yellow'];

foreach ($colors as $color) {
    echo $color . PHP_EOL;
}
  1. Run the script:
bash
php control-structures.php
  1. Loop through keys and values:
php
<?php

$user = [
    'name' => 'Dale',
    'email' => 'dale@example.com',
    'role' => 'Admin'
];

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

Expected Result ​

Red
Green
Blue
Yellow
name: Dale
email: dale@example.com
role: Admin

Why It Works ​

  • foreach automatically assigns each array value to $color on each iteration.
  • Using the as $key => $value syntax provides access to both the key and the value without manual indexing.
  • PHP_EOL ensures each item prints on its own line across different operating systems.

Troubleshooting ​

  • Undefined index errors — These occur when using for with a numeric counter. foreach avoids this by iterating over actual elements.
  • Accidentally modifying the original array — Remember that iterating by reference (e.g., foreach ($array as &$value)) alters the original array. Use references only when you intend to mutate the data.

Step 5: Alternative Conditional Structures (~6 min) ​

Goal ​

Compare switch, match, and other concise conditional operators to choose the right tool for each scenario.

Actions ​

  1. Use switch for multiple value comparisons:
php
<?php

$day = 'Tuesday';

switch ($day) {
    case 'Monday':
        echo "Start of the work week!" . PHP_EOL;
        break;
    case 'Tuesday':
    case 'Wednesday':
    case 'Thursday':
        echo "Midweek grind." . PHP_EOL;
        break;
    case 'Friday':
        echo "Almost weekend!" . PHP_EOL;
        break;
    case 'Saturday':
    case 'Sunday':
        echo "Weekend!" . PHP_EOL;
        break;
    default:
        echo "Not a valid day." . PHP_EOL;
        break;
}

  1. Run the script to observe the output.

  2. Experiment with match (PHP 8.0+):

php
<?php

$statusCode = 404;

$message = match ($statusCode) {
    200 => "Success",
    201 => "Created",
    400 => "Bad Request",
    401 => "Unauthorized",
    404 => "Not Found",
    500 => "Server Error",
    default => "Unknown Status",
};

echo "HTTP $statusCode: $message" . PHP_EOL;
  1. Apply concise operators like the ternary and null coalescing operators:
php
<?php

$age = 20;
$status = ($age >= 18) ? 'Adult' : 'Minor';
echo "Status: $status" . PHP_EOL;

$ticketPrice = ($age < 12) ? 5 : (($age >= 65) ? 8 : 12);
echo "Ticket price: $$ticketPrice" . PHP_EOL;

$username = null;
$displayName = $username ?? 'Guest';
echo "Welcome, $displayName!" . PHP_EOL;

Expected Result ​

Midweek grind.
HTTP 404: Not Found
Status: Adult
Ticket price: $12
Welcome, Guest!

Why It Works ​

  • switch is useful when comparing a single value against many discrete options. Remember to include break statements to prevent fall-through.
  • match uses strict comparison (===), returns a value, and doesn’t require break statements. It’s ideal when you need an expression rather than statements.
  • The ternary operator offers a concise alternative to simple if/else checks, while the null coalescing operator (??) provides default values when variables are null or undefined.

Troubleshooting ​

  • switch executes multiple cases — Ensure each case ends with break unless you intentionally want fall-through.
  • match throws UnhandledMatchError — Provide a default arm to handle unexpected values.
  • Nested ternaries are unreadable — If the condition becomes complex, switch back to standard if/else or use match for clarity.

Step 6: Controlling Loop Flow with break and continue (~3 min) ​

Goal ​

Exit loops early or skip specific iterations using break and continue.

Actions ​

  1. Use break to exit a loop when a condition is met:
php
<?php

// Find the first number divisible by 7
for ($i = 1; $i <= 100; $i++) {
    if ($i % 7 === 0) {
        echo "Found it! The first number divisible by 7 is: $i" . PHP_EOL;
        break; // Exit the loop immediately
    }
}
  1. Use continue to skip specific iterations:
php
<?php

// Print odd numbers from 1 to 10
for ($i = 1; $i <= 10; $i++) {
    if ($i % 2 === 0) {
        continue; // Skip even numbers
    }
    echo $i . PHP_EOL;
}

Expected Result ​

Found it! The first number divisible by 7 is: 7
1
3
5
7
9

Why It Works ​

  • break stops the loop entirely as soon as the condition is satisfied, saving unnecessary iterations.
  • continue skips the rest of the current loop body and moves on to the next iteration.
  • Combining these tools allows fine-grained control over loop execution without nesting additional conditionals.

Troubleshooting ​

  • Loop still runs after break — Confirm the break statement is inside the correct loop, especially when loops are nested.
  • continue appears to skip more iterations than expected — Check the condition; ensure the code after continue isn’t handling required work, and verify the loop counter still increments.
  • Inconsistent results — Add debug statements before and after break/continue to trace the execution flow.

Troubleshooting Common Issues ​

Goal ​

Identify and fix the most common pitfalls when working with control structures.

Actions ​

Review these scenarios and apply the suggested fixes when you run into similar behavior.

Issue: Infinite Loop ​

Symptom: Your script runs forever and never completes.

Cause: The loop condition never becomes false.

Solution: Ensure your loop has a way to exit. Press Ctrl+C in the terminal to stop a runaway script and then fix the logic.

php
// BAD: This will run forever
$i = 1;
while ($i <= 10) {
    echo $i . PHP_EOL;
    // Forgot to increment $i!
}

// GOOD: Loop will exit
$i = 1;
while ($i <= 10) {
    echo $i . PHP_EOL;
    $i++; // This eventually makes the condition false
}

Issue: Comparison Operator Confusion ​

Symptom: Your if statement isn't working as expected.

Cause: Using = (assignment) instead of == or === (comparison).

Solution: Always use === for comparisons so types must match.

php
// BAD: This assigns 5 to $age, doesn't compare!
if ($age = 5) {
    echo "You are 5 years old.";
}

// GOOD: This compares $age to 5
if ($age === 5) {
    echo "You are 5 years old.";
}

Issue: Unexpected Array Loop Behavior ​

Symptom: Your foreach loop modifies the array but changes don't persist.

Cause: By default, foreach works with a copy of each element.

Solution: Use references only when you want to mutate the array, and remember to unset the reference afterward.

php
$numbers = [1, 2, 3];

// This doesn't modify the original array
foreach ($numbers as $num) {
    $num = $num * 2;
}
// $numbers is still [1, 2, 3]

// This DOES modify the original array
foreach ($numbers as &$num) {
    $num = $num * 2;
}
unset($num); // Important! Remove the lingering reference
// $numbers is now [2, 4, 6]

Issue: Switch Fall-through ​

Symptom: Your switch statement executes multiple cases when you only wanted one.

Cause: Missing break statement causes execution to "fall through" to the next case.

Solution: Always add break after each case (unless you intentionally want fall-through).

php
// BAD: Missing break statements
$color = 'blue';
switch ($color) {
    case 'red':
        echo "Red";
    case 'blue':
        echo "Blue";
    case 'green':
        echo "Green";
}
// Output: BlueGreen (falls through!)

// GOOD: Proper break statements
$color = 'blue';
switch ($color) {
    case 'red':
        echo "Red";
        break;
    case 'blue':
        echo "Blue";
        break;
    case 'green':
        echo "Green";
        break;
}
// Output: Blue

Exercises ​

  1. FizzBuzz: This is a classic programming challenge. Write a script that prints the numbers from 1 to 100.

    • For multiples of three, print "Fizz" instead of the number.
    • For multiples of five, print "Buzz" instead of the number.
    • For numbers which are multiples of both three and five, print "FizzBuzz".

  2. Multiplication Table: Create a script that prints a multiplication table for a given number (e.g., 7) up to 12.

    • Example output for the number 7:
      7 x 1 = 7
7 x 2 = 14
...
7 x 12 = 84

  3. Number Guessing Game: Create a script where the program picks a random number between 1 and 20, and you have to guess it. After each guess, the program should tell you if your guess is too high, too low, or correct. Hint: You'll need to use a while loop and comparison operators.

  4. Grade Calculator: Create a script that converts numerical scores to letter grades using a match expression.

    • 90-100: A
    • 80-89: B
    • 70-79: C
    • 60-69: D
    • Below 60: F
    • Hint: You can use match with conditions like $score >= 90 => 'A'

Further Reading ​

To dive deeper into control structures, check out these official PHP documentation pages:

Wrap-up ​

You've just learned one of the most powerful concepts in programming: controlling the flow of your code. You can now:

  • Make decisions using if, elseif, and else statements
  • Combine conditions with logical operators (&&, ||, !)
  • Use switch for multiple conditions and modern match expressions (PHP 8.0+)
  • Write concise conditionals with the ternary operator (? :)
  • Handle null values elegantly with the null coalescing operator (??)
  • Repeat tasks with for, while, and foreach loops
  • Control loop execution with break and continue
  • Troubleshoot common control structure issues

These building blocks are essential for writing any meaningful program. You'll use them in virtually every script you write from this point forward. The modern features like match and ?? are particularly powerful tools that make PHP 8.4 code cleaner and more expressive.

In the next chapter, we'll learn how to bundle our code into reusable blocks called functions, making our programs much more organized and powerful.

Code Examples

Complete, runnable examples from this chapter are available in:

Knowledge Check ​

Test your understanding of control structures:

Chapter 03 Quiz: Control Structures

Question 1 of 5
What is the difference between == and === in PHP?
  • === checks both value and type, while == only checks value
  • They are exactly the same
  • === is faster than ==
  • == is stricter than ===