PHP array_diff_ukey Function: How it Works with Examples

The PHP array_diff_ukey function compares keys in two or more arrays. It returns the keys from the first array that do not match keys in the other arrays.

You can pass a custom callback to decide how the keys match. This function is useful when arrays have special key formats and need strict custom comparison.

Syntax and Parameters of array_diff_ukey in PHP Function

The syntax looks like this:

array_diff_ukey(array $array1, array $array2, array ...$arrays, callable $key_compare_func)

Here is how it works:

• array1: the main array to compare.
• array2, …arrays: arrays to compare with the first array.
• key_compare_func: a user function that defines the logic of key comparison.

The function checks only the keys. It does not check values. The comparison logic depends on the callback you provide.

Here is a quick example:

function compareKeys($a, $b) {
    return strcmp($a, $b);
}

$result = array_diff_ukey(
    ["a" => "red", "b" => "green", "c" => "blue"],
    ["c" => "yellow", "d" => "black"],
    "compareKeys"
);

print_r($result);

This returns the keys a and b because they do not exist in the second array. The output:

Array
(
    [a] => red
    [b] => green
)

The Difference Between array_diff_ukey and array_diff_key

Both functions compare array keys. The difference is in how they test equality.

• array_diff_key: compares keys with default PHP comparison rules.
• array_diff_ukey: compares keys with a custom function.

Here is the comparison:

You can use the array_diff_key if arrays need simple key comparison, while the array_diff_ukey if arrays need special rules for keys.

Examples of array_diff_ukey in PHP

Basic Custom Key Comparison:

function myCompare($a, $b) {
    return strcmp($a, $b);
}

$num1 = ["X" => 10, "Y" => 20, "Z" => 30];
$num2 = ["Y" => 40, "A" => 50];

print_r(array_diff_ukey($num1, $num2, "myCompare"));

This compares the keys with a custom function. The result keeps keys X and Z because they do not exist in the second array. Here is the output:

Array
(
    [X] => 10
    [Z] => 30
)

Numeric Keys with Custom Logic:

function compareNumbers($a, $b) {
    return $a <=> $b;
}

$array1= [1 => "cat", 2 => "dog", 3 => "bird"];
$array2= [3 => "fish", 4 => "tiger"];

print_r(array_diff_ukey($array1, $array2, "compareNumbers"));

This compares numeric keys with a callback. Keys 1 and 2 remain because they do not match keys from the second array. The output:

Array
(
    [1] => cat
    [2] => dog
)

Case-Insensitive Comparison:

function compareCase($a, $b) {
    return strcasecmp($a, $b);
}

$values1 = ["One" => 1, "Two" => 2, "Three" => 3];
$values2 = ["one" => 11, "four" => 44];

print_r(array_diff_ukey($values1, $values2, "compareCase"));

This compares keys without case sensitivity. Keys Two and Three remain because the function treats One and one as equal. The output:

Array
(
    [Two] => 2
    [Three] => 3
)

Multi-Array Key Comparison:

function extract_array_keys_callback($a, $b) {
    return strcmp($a, $b);
}

$list1 = array(
  "a" => 1, 
  "b" => 2, 
  "c" => 3
);

$arra2 = array(
  "c" => 10, 
  "d" => 20
);

$numeric3 = array(
  "a" => 100, 
  "e" => 200
);

$keys_diff = array_diff_ukey($list1, $arra2, $numeric3, "extract_array_keys_callback");

print_r( $keys_diff );

This checks the keys in all three arrays. Only b stays because the a key and c key appear in the other. The output:

Array
(
    [b] => 2
)

Wrapping Up

You learned what array_diff_ukey does and how it compares keys with a custom callback.

Here is a quick recap:

• array_diff_ukey compares only keys, not values.
• You can pass a custom function to compare the keys of arrays.
• array_diff_key uses normal PHP comparison rules.
• It works with special logic for complex cases.

FAQs

function compare_keys($a, $b) {
    if ($a === $b) return 0;
    return ($a > $b) ? 1 : -1;
}
$array1 = array("a" => "apple", "b" => "banana", "c" => "cherry");
$array2 = array("a" => "apple", "d" => "date");
$result = array_diff_ukey($array1, $array2, "compare_keys");
print_r($result);

$array1 = array("a" => "apple", "b" => "banana");
$array2 = array("a" => "apple", "c" => "cherry");

print_r(array_diff_key($array1, $array2));
print_r(array_diff_ukey($array1, $array2, function($a,$b){
    return strcmp($a,$b);
}));

function case_insensitive($a, $b) {
    return strcasecmp($a, $b);
}
$array1 = array("A" => "apple", "B" => "banana");
$array2 = array("a" => "apricot");
$result = array_diff_ukey($array1, $array2, "case_insensitive");
print_r($result);

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