PHP Nested Loops: A Loop Inside a Loop

A loop can contain another loop. The inner loop runs completely for every single round of the outer loop.

index.php

PHP

<?php
for ($row = 1; $row <= 3; $row++) {
    for ($col = 1; $col <= 3; $col++) {
        echo "($row,$col) ";
    }
    echo "\n";
}

Output

OUTPUT

1
2
3

(1,1) (1,2) (1,3)
(2,1) (2,2) (2,3)
(3,1) (3,2) (3,3)

The outer loop runs 3 times. Each time, the inner loop runs 3 times, so the inside code runs 9 times in total.

A multiplication table

index.php

PHP

<?php
for ($i = 1; $i <= 4; $i++) {
    for ($j = 1; $j <= 4; $j++) {
        echo str_pad($i * $j, 4, " ", STR_PAD_LEFT);
    }
    echo "\n";
}

Output

OUTPUT

   1   2   3   4
   2   4   6   8
   3   6   9  12
   4   8  12  16

A star pyramid

index.php

PHP

<?php
for ($i = 1; $i <= 4; $i++) {
    echo str_repeat(" ", 4 - $i) . str_repeat("*", $i * 2 - 1) . "\n";
}

Output

OUTPUT

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

Nested arrays

This is the most common real use.

index.php

PHP

<?php
$classes = [
    "Class A" => ["Mia", "Noah"],
    "Class B" => ["Olivia", "Pete", "Quinn"],
];

foreach ($classes as $className => $students) {
    echo $className . ":\n";

    foreach ($students as $student) {
        echo "  - $student\n";
    }
}

Output

OUTPUT

Class A:
  - Mia
  - Noah
Class B:
  - Olivia
  - Pete
  - Quinn

Build an HTML table

index.php

PHP

<?php
$rows = [["Pen", 10], ["Book", 250]];

echo "<table>";
foreach ($rows as $row) {
    echo "<tr>";
    foreach ($row as $cell) {
        echo "<td>$cell</td>";
    }
    echo "</tr>";
}
echo "</table>";

Output

OUTPUT

<table><tr><td>Pen</td><td>10</td></tr><tr><td>Book</td><td>250</td></tr></table>

Watch the cost

Nested loops multiply work. Two loops over 1,000 items run 1,000,000 times. For large data, try to avoid nesting, for example by looking items up in an array by key instead of searching with an inner loop. See Avoiding Repeated Work.

lightbulb

Use different counter names

Name loop variables for what they mean ($row, $col, $student). Using $i and $j everywhere makes nested loops hard to read.