PHP array_udiff_assoc Function

last modified March 13, 2025

The PHP array_udiff_assoc function computes the difference of arrays with additional index check. It compares data using a callback function.

Basic Definition

array_udiff_assoc compares array values with a callback while checking keys. It returns values from array1 not present in any other arrays.

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

Basic array_udiff_assoc Example

This shows a simple comparison of two arrays with both value and key checks.

basic_array_udiff_assoc.php

<?php

declare(strict_types=1);

function compare_values($a, $b): int {
    return $a <=> $b;
}

$array1 = ["a" => 1, "b" => 2, "c" => 3];
$array2 = ["a" => 1, "b" => 5, "d" => 4];

$result = array_udiff_assoc($array1, $array2, 'compare_values');

print_r($result);

This compares arrays by both keys and values. Only elements with matching keys and values are considered equal. The callback performs standard comparison.

Comparing Objects

Use array_udiff_assoc to compare arrays containing objects.

object_comparison.php

<?php

declare(strict_types=1);

class Product {
    public function __construct(
        public string $name,
        public float $price
    ) {}
}

function compare_products($a, $b): int {
    return $a->price <=> $b->price;
}

$products1 = [
    "p1" => new Product("Laptop", 999.99),
    "p2" => new Product("Phone", 699.99)
];

$products2 = [
    "p1" => new Product("Laptop", 999.99),
    "p3" => new Product("Tablet", 399.99)
];

$diff = array_udiff_assoc($products1, $products2, 'compare_products');

print_r($diff);

This compares Product objects by price while checking array keys. Only products with both matching keys and equal prices are considered the same.

Case-Insensitive String Comparison

Perform case-insensitive comparison of string values while checking keys.

case_insensitive.php

<?php

declare(strict_types=1);

function case_insensitive_compare($a, $b): int {
    return strcasecmp($a, $b);
}

$array1 = ["A" => "Apple", "B" => "Banana"];
$array2 = ["a" => "apple", "b" => "berry"];

$result = array_udiff_assoc($array1, $array2, 'case_insensitive_compare');

print_r($result);

This compares string values case-insensitively while maintaining strict key comparison. Keys must match exactly, but values are compared without case.

Multiple Array Comparison

Compare one array against multiple other arrays with key and value checks.

multiple_arrays.php

<?php

declare(strict_types=1);

function numeric_compare($a, $b): int {
    return $a - $b;
}

$main = ["x" => 10, "y" => 20, "z" => 30];
$array1 = ["x" => 10, "y" => 25];
$array2 = ["x" => 15, "z" => 30];

$diff = array_udiff_assoc($main, $array1, $array2, 'numeric_compare');

print_r($diff);

This finds elements in $main not present in any other arrays, comparing both keys and values. Only "y" => 20 meets this condition in the example.

Complex Custom Comparison

Implement a custom comparison logic for specialized array difference checks.

custom_comparison.php

<?php

declare(strict_types=1);

function complex_compare($a, $b): int {
    if (is_numeric($a) && is_numeric($b)) {
        return ($a > $b) ? 1 : (($a < $b) ? -1 : 0);
    }
    return strcmp((string)$a, (string)$b);
}

$array1 = ["a" => "10", "b" => 20, "c" => "30"];
$array2 = ["a" => 10, "b" => "20", "d" => 40];

$result = array_udiff_assoc($array1, $array2, 'complex_compare');

print_r($result);

This implements a custom comparison that handles numeric strings differently. The function treats numeric strings and numbers as equal if their values match.

Best Practices

Consistent Callbacks: Ensure your callback returns consistent comparison results.
Type Safety: Add type hints to callback parameters when possible.
Performance: For large arrays, optimize your comparison logic.
Key Awareness: Remember keys must match exactly in addition to value comparison.

Source

PHP array_udiff_assoc Documentation

This tutorial covered the PHP array_udiff_assoc 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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