Loops in PHP

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PHP Loops(DOCUMENTATION)

Introduction

Loops let you execute code repeatedly, which is essential for processing collections, generating output, and running tasks until a condition is met. This lesson covers all four PHP loop types and the control statements (break, continue) that manage loop flow.

Key Concepts

  • for Loop: Best when you know the exact number of iterations ahead of time.
  • while Loop: Continues as long as a condition remains true. The condition is checked before each iteration.
  • do-while Loop: Like while, but the body executes at least once because the condition is checked after.
  • foreach Loop: Designed specifically for iterating over arrays and objects. The most commonly used loop in PHP.
  • break and continue: Control statements that exit a loop or skip to the next iteration.

Real World Context

Processing database results, iterating over form inputs, rendering lists of products, paginating API responses — loops are everywhere in web development. The foreach loop is by far the most common in PHP because PHP applications work heavily with arrays (query results, JSON data, configuration arrays). Understanding break and continue helps you write efficient loops that skip unnecessary work.

Deep Dive

for Loop

Best when you know the number of iterations upfront:

php
<?php
for ($i = 0; $i < 5; $i++) {
    echo "Iteration: $i\n";
}
// Outputs: 0, 1, 2, 3, 4

The syntax is for (init; condition; increment). The init runs once, the condition is checked before each iteration, and the increment runs after each iteration.

while Loop

Continue while a condition is true:

php
<?php
$count = 0;

while ($count < 3) {
    echo "Count: $count\n";
    $count++;
}
// Outputs: 0, 1, 2

If the condition is false initially, the body never executes.

do-while Loop

Executes the body at least once, then checks the condition:

php
<?php
$x = 10;

do {
    echo "x is: $x\n";
    $x++;
} while ($x < 5);
// Outputs: "x is: 10" (runs once even though 10 > 5)

This is useful for menus, retry logic, or any scenario where you need at least one execution.

foreach Loop

The go-to loop for arrays and objects:

php
<?php
$fruits = ['apple', 'banana', 'cherry'];

// Value only
foreach ($fruits as $fruit) {
    echo "$fruit\n";
}

// Key and value
foreach ($fruits as $index => $fruit) {
    echo "$index: $fruit\n";
}

// Associative array
$user = ['name' => 'John', 'age' => 30];
foreach ($user as $key => $value) {
    echo "$key = $value\n";
}

The foreach loop handles both indexed and associative arrays seamlessly.

Loop Control: break and continue

break exits the loop entirely:

php
<?php
for ($i = 0; $i < 10; $i++) {
    if ($i === 5) {
        break;  // Stop at 5
    }
    echo $i;
}
// Outputs: 01234

continue skips to the next iteration:

php
<?php
for ($i = 0; $i < 5; $i++) {
    if ($i === 2) {
        continue;  // Skip 2
    }
    echo $i;
}
// Outputs: 0134

Breaking Nested Loops

Pass a number to break to exit multiple levels:

php
<?php
for ($i = 0; $i < 3; $i++) {
    for ($j = 0; $j < 3; $j++) {
        if ($j === 1) {
            break 2;  // Break out of both loops
        }
    }
}

The number after break specifies how many nested loops to exit.

Modifying Arrays in Foreach

Use a reference (&) to modify array elements in place:

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

foreach ($numbers as &$num) {
    $num *= 2;
}
unset($num);  // Important! Break the reference

print_r($numbers);  // [2, 4, 6]

Always unset() the reference variable after the loop to prevent unexpected behavior in later code.

Common Pitfalls

  1. Forgetting to unset() after a reference foreach — The $num variable remains a reference to the last array element. Reusing it later will silently modify the array.
  2. Infinite loops — Forgetting to increment the counter in a while loop creates an infinite loop. Always ensure the condition will eventually become false.
  3. Modifying an array while iterating — Adding or removing elements during a foreach leads to unpredictable results. Collect changes in a separate array and apply them after the loop.

Best Practices

  1. Prefer foreach for arrays — It is cleaner, safer, and more idiomatic than using for with manual index management.
  2. Extract complex loop bodies into functions — If your loop body exceeds 10-15 lines, move the logic into a separate function for readability.
  3. Use break with early returns — When searching for a value, break as soon as you find it. Do not iterate through the rest of the array unnecessarily.

Summary

  • for loops are for known iteration counts; while and do-while are for condition-based loops.
  • foreach is the most common loop in PHP, designed for arrays and objects.
  • break exits a loop; continue skips to the next iteration.
  • break 2 exits two nested loops at once.
  • Always unset() reference variables after a foreach with &.

Code Examples

php
<?php
// Real example: Processing a list of orders
$orders = [
    ['id' => 1, 'total' => 99.99, 'status' => 'pending'],
    ['id' => 2, 'total' => 149.50, 'status' => 'shipped'],
    ['id' => 3, 'total' => 25.00, 'status' => 'pending'],
    ['id' => 4, 'total' => 200.00, 'status' => 'delivered'],
];

$pendingTotal = 0;
$pendingCount = 0;

foreach ($orders as $order) {
    if ($order['status'] !== 'pending') {
        continue;  // Skip non-pending orders
    }

    $pendingTotal += $order['total'];
    $pendingCount++;
}

echo "Pending orders: $pendingCount\n";
echo "Pending total: \$$pendingTotal\n";
// Pending orders: 2
// Pending total: $124.99
?>
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