PHP array_intersect_uassoc Function

last modified March 13, 2025

The PHP array_intersect_uassoc function computes the intersection of arrays with additional index check. It compares keys using a callback.

Basic Definition

array_intersect_uassoc returns an array containing all values from the first array that are present in all other arrays. Keys are compared using a user-supplied callback function.

Syntax: array_intersect_uassoc(array $array1, array $array2, ..., callable $key_compare_func): array. The callback should return an integer less than, equal to, or greater than zero.

Basic array_intersect_uassoc Example

This example shows how to find intersection of arrays with case-insensitive key comparison.

basic_array_intersect_uassoc.php

<?php

declare(strict_types=1);

$array1 = ["a" => "apple", "B" => "banana", "c" => "cherry"];
$array2 = ["A" => "apple", "b" => "banana", "C" => "cherry"];

$result = array_intersect_uassoc($array1, $array2, function($a, $b) {
    return strcasecmp($a, $b);
});

print_r($result);

This finds elements present in both arrays where keys match case-insensitively. The callback uses strcasecmp for case-insensitive comparison.

Numeric Key Comparison

Compare numeric keys with custom logic to find matching elements.

numeric_key_comparison.php

<?php

declare(strict_types=1);

$array1 = [1 => "one", 2 => "two", 3 => "three"];
$array2 = ["1" => "one", "2" => "two", 4 => "four"];

$result = array_intersect_uassoc($array1, $array2, function($a, $b) {
    return $a <=> $b;
});

print_r($result);

This compares numeric keys loosely (string vs integer). The spaceship operator (<=>) handles the comparison, matching keys 1 and 2.

Complex Key Comparison

Use a more complex callback function to compare composite keys.

complex_key_comparison.php

<?php

declare(strict_types=1);

$array1 = ["user_1" => "Alice", "user_2" => "Bob", "admin_1" => "Charlie"];
$array2 = ["USER_1" => "Alice", "user_3" => "Dave", "ADMIN_1" => "Charlie"];

$result = array_intersect_uassoc($array1, $array2, function($a, $b) {
    $aParts = explode('_', strtolower($a));
    $bParts = explode('_', strtolower($b));
    
    if ($aParts[0] !== $bParts[0]) return $aParts[0] <=> $bParts[0];
    return $aParts[1] <=> $bParts[1];
});

print_r($result);

This compares keys by splitting them into parts and comparing each part. The callback normalizes case and compares type then ID separately.

Multiple Array Comparison

Compare more than two arrays with custom key comparison logic.

multiple_array_comparison.php

<?php

declare(strict_types=1);

$array1 = ["a" => "apple", "b" => "banana", "c" => "cherry"];
$array2 = ["A" => "apple", "B" => "banana", "D" => "date"];
$array3 = ["a" => "apple", "b" => "blueberry", "c" => "cherry"];

$result = array_intersect_uassoc($array1, $array2, $array3, function($a, $b) {
    return strcasecmp($a, $b);
});

print_r($result);

This finds elements present in all three arrays with case-insensitive key matching. Only "apple" with key "a" appears in all arrays.

Object Key Comparison

Compare arrays with object keys using a custom comparison function.

object_key_comparison.php

<?php

declare(strict_types=1);

class User {
    public function __construct(public int $id) {}
}

$user1 = new User(1);
$user2 = new User(2);
$user3 = new User(1);

$array1 = [$user1 => "Alice", $user2 => "Bob"];
$array2 = [$user3 => "Alice", $user2 => "Charlie"];

$result = array_intersect_uassoc($array1, $array2, function($a, $b) {
    return $a->id <=> $b->id;
});

print_r($result);

This compares objects as keys by their ID property. The callback uses the spaceship operator to compare IDs, matching User 1 in both arrays.

Best Practices

Consistent Callbacks: Ensure your callback provides consistent comparisons.
Type Safety: Add type hints to callback parameters when possible.
Performance: Keep key comparison logic efficient for large arrays.
Documentation: Clearly document custom comparison logic.

Source

PHP array_intersect_uassoc Documentation

This tutorial covered the PHP array_intersect_uassoc function with practical examples showing its usage for array intersection with custom key comparison.

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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