PHP array_diff Function

last modified March 13, 2025

The PHP array_diff function compares arrays and returns the differences. It's useful for finding values that exist in one array but not others.

Basic Definition

The array_diff function compares array values and returns a new array containing entries from the first array not present in others.

Syntax: array_diff(array $array, array ...$arrays): array. It compares values using loose comparison (==). Keys are preserved in the result.

Basic array_diff Example

This shows a simple comparison between two arrays to find unique values.

basic_array_diff.php

<?php

$array1 = ["a", "b", "c", "d"];
$array2 = ["b", "c", "e"];

$result = array_diff($array1, $array2);

print_r($result);
// Output: Array ( [0] => a [3] => d )

This finds values in $array1 not present in $array2. The result contains "a" and "d" with their original indices preserved from $array1.

Comparing Multiple Arrays

array_diff can compare against multiple arrays at once.

multi_array_diff.php

<?php

$mainArray = [1, 2, 3, 4, 5, 6];
$arrayA = [2, 4];
$arrayB = [3, 5];

$result = array_diff($mainArray, $arrayA, $arrayB);

print_r($result);
// Output: Array ( [0] => 1 [5] => 6 )

This finds values in $mainArray not present in either $arrayA or $arrayB. The result contains 1 and 6, the only numbers unique to $mainArray.

Associative Array Comparison

array_diff works with associative arrays, comparing only values.

assoc_array_diff.php

<?php

$user1 = ["name" => "John", "age" => 30, "city" => "New York"];
$user2 = ["name" => "Jane", "age" => 25, "city" => "New York"];

$result = array_diff($user1, $user2);

print_r($result);
// Output: Array ( [name] => John [age] => 30 )

This compares associative arrays by value. The result shows differences in name and age fields. The city value is the same so it's excluded from the result.

Strict Comparison with array_diff

For strict comparison (===), combine array_diff with array_udiff.

strict_array_diff.php

<?php

$array1 = ["1", 2, 3];
$array2 = [1, 2, "3"];

// Regular array_diff (loose comparison)
$looseDiff = array_diff($array1, $array2);

// Strict comparison
$strictDiff = array_udiff($array1, $array2, function($a, $b) {
    return $a === $b ? 0 : 1;
});

print_r($looseDiff);   // Output: Array ( )
print_r($strictDiff);  // Output: Array ( [0] => 1 [2] => 3 )

This demonstrates the difference between loose and strict comparison. The regular array_diff finds no differences while the strict version does, due to type differences between string "1" and integer 1.

Finding New Items in Updated Data

Practical example: finding new items in an updated dataset compared to original.

practical_array_diff.php

<?php

$originalProducts = ["Laptop", "Phone", "Tablet"];
$updatedProducts = ["Laptop", "Phone", "Tablet", "Watch", "Headphones"];

$newProducts = array_diff($updatedProducts, $originalProducts);

print_r($newProducts);
// Output: Array ( [3] => Watch [4] => Headphones )

This identifies new products added to the inventory. The result contains "Watch" and "Headphones" which weren't in the original product list.

Best Practices

Key Preservation: Be aware that keys are preserved from the first array.
Performance: For large arrays, consider sorting first for better performance.
Type Awareness: Remember it uses loose comparison by default.
Order Matters: The first array determines which values are checked.

Source

PHP array_diff Documentation

This tutorial covered the PHP array_diff function with practical examples showing its usage for array comparison scenarios.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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