13. PHP Loops

What you will learn

How to repeat code with for, while, and do...while
How to loop through arrays with foreach
How to use break and continue
How to avoid endless loops

Why loops?

A loop runs the same code several times. Write the code once, and let PHP repeat it.

The for loop

PHP code

<?php
for ($i = 1; $i <= 5; $i++) {
    echo "Round $i<br>";
}

What the browser shows

Round 1
Round 2
Round 3
Round 4
Round 5

The header has three parts: a start ($i = 1), a condition checked before each round ($i <= 5), and a step after each round ($i++).

The while loop

Use while when you do not know in advance how many rounds are needed.

PHP code

<?php
$money = 100;
$years = 0;

while ($money < 200) {
    $money = $money * 1.1;
    $years++;
}

echo "It takes $years years to double.";

Output

It takes 8 years to double.

The do…while loop

It checks the condition at the end, so the body always runs at least once.

PHP code

<?php
$n = 10;

do {
    echo "n is $n";
} while ($n < 5);

Output

n is 10

The foreach loop

foreach is the loop you will use most. It goes through every item of an array.

PHP code

<?php
$courses = ["HTML", "CSS", "JavaScript", "PHP"];

foreach ($courses as $course) {
    echo "I am learning $course<br>";
}

What the browser shows

I am learning HTML
I am learning CSS
I am learning JavaScript
I am learning PHP

To get the key as well, write foreach ($array as $key => $value). You will use this with associative arrays.

break and continue

break leaves the loop completely.
continue skips the rest of this round and starts the next.

PHP code

<?php
for ($i = 1; $i <= 10; $i++) {
    if ($i % 2 === 0) { continue; }
    if ($i > 7) { break; }
    echo $i . " ";
}

Output

1 3 5 7

Loops inside HTML

The colon syntax with endforeach keeps templates tidy.

PHP code

<?php $tools = ["Editor", "Browser", "Terminal"]; ?>

<ul>
<?php foreach ($tools as $tool): ?>
  <li><?= htmlspecialchars($tool) ?></li>
<?php endforeach; ?>
</ul>

What the browser shows

Editor
Browser
Terminal

Nested loops

PHP code

<?php
for ($row = 1; $row <= 3; $row++) {
    for ($col = 1; $col <= 4; $col++) {
        echo "*";
    }
    echo "<br>";
}

What the browser shows

****
****
****

Endless loops: if the condition never becomes false, the script runs until PHP’s time limit stops it. Make sure something inside the loop changes the condition.

Common mistakes

Forgetting $i++, which causes an endless loop.
Off-by-one errors with < versus <=.
Using for with array indexes when foreach is simpler.
Changing an array while looping over it.
Printing visitor data in a loop without htmlspecialchars().

Exercise

Use a loop to add up the numbers from 1 to 10 and print Total: 55.

Show solution

<?php
$total = 0;

for ($i = 1; $i <= 10; $i++) {
    $total += $i;
}

echo "Total: $total";

Knowledge check

1. Which loop goes through every item of an array?

foreach

2. What does continue do?

It skips to the next round of the loop.

3. Which loop always runs at least once?

do...while

Summary

for suits a known count, while an unknown one, and foreach arrays.
break exits and continue skips a round.
Always make sure a loop can finish.