How can the array_udiff() function in PHP be used to compare multidimensional arrays and retrieve unique elements?

When comparing multidimensional arrays in PHP and retrieving unique elements, the array_udiff() function can be used along with a custom comparison function. This function will determine the uniqueness of elements based on a specified criteria. By passing this custom function to array_udiff(), you can compare the arrays and retrieve elements that are unique across all arrays. 

// Custom comparison function to compare multidimensional arrays based on a specific criteria
function compareArrays($a, $b) {
    return $a['key'] <=> $b['key'];
}

// Multidimensional arrays to compare
$array1 = [
    ['key' => 1, 'value' => 'A'],
    ['key' => 2, 'value' => 'B'],
    ['key' => 3, 'value' => 'C']
];

$array2 = [
    ['key' => 2, 'value' => 'B'],
    ['key' => 3, 'value' => 'C'],
    ['key' => 4, 'value' => 'D']
];

// Retrieve unique elements by comparing arrays using array_udiff()
$uniqueElements = array_udiff($array1, $array2, 'compareArrays');

// Output unique elements
print_r($uniqueElements);

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