PHP array_intersect_ukey Function: How it Works with Examples

The array_intersect_ukey is used to compare arrays by keys with a custom function in PHP. It returns matches by keys only.

Understand the array_intersect_ukey in PHP

The array_intersect_ukey function compares keys of two or more arrays. It checks matches with a user-defined compare function.

The syntax looks like this:

array_intersect_ukey(array $array1, array $array2, callable $key_compare_func)

Here is how it works:

• array1: The first array.
• array2: The second array.
• key_compare_func: A function that decides how to compare keys.

The function runs the compare function for each key pair. It keeps only keys that match.

For a quick example:

function compareKeys($a, $b) {
    if ($a === $b) return 0;
    return ($a > $b) ? 1 : -1;
}

$arr1 = ["a" => "red", "b" => "blue", "c" => "green"];
$arr2 = ["a" => "yellow", "b" => "pink", "d" => "black"];

$result = array_intersect_ukey($arr1, $arr2, "compareKeys");
print_r($result);

This code compares keys with compareKeys. It finds only keys that match both arrays. The output will be:

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

So, how does the user-defined compare function work?

The compare function must return:

• 0 if keys are equal.
• 1 if the first key is greater.
• -1 if the first key is smaller.

PHP uses this return value to decide if keys match.

The Difference Between array_intersect and array_intersect_ukey

The array_intersect function:

This function compares array values. It keeps only values that appear in all arrays.

The array_intersect_ukey function:

This function compares array keys. It needs a user-defined compare function to match keys.

Here is a table that shows you the key differences:

Use array_intersect to find common values. Use array_intersect_ukey to find common keys.

Examples of array_intersect_ukey in PHP

Match Keys in Two Arrays

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

$arr1 = ["x" => 10, "y" => 20, "z" => 30];
$arr2 = ["x" => 15, "y" => 25, "w" => 40];

$result = array_intersect_ukey($arr1, $arr2, "compareKeys");
print_r($result);

This example matches keys x and y only. It ignores values. The output is ["x" => 10, "y" => 20]. This shows that the function filters by keys alone.

Compare Three Arrays by Keys:

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

$arr1 = ["a" => 1, "b" => 2, "c" => 3];
$arr2 = ["a" => 10, "b" => 20, "d" => 40];
$arr3 = ["a" => 100, "b" => 200, "e" => 500];

$result = array_intersect_ukey($arr1, $arr2, $arr3, "keyCompare");
print_r($result);

This example uses three arrays. It keeps only keys a and b because they appear in all arrays.

The output:

Array
(
    [a] => 1
    [b] => 2
)

Custom Sort Logic in Compare Function:

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

$arr1 = ["c" => 3, "b" => 2, "a" => 1];
$arr2 = ["a" => 5, "b" => 6, "c" => 7];

$result = array_intersect_ukey($arr1, $arr2, "reverseCompare");
print_r($result);

This example compares keys with reverse logic. All three keys match, so the result includes all pairs, and the compare function changes order logic but the match still happens.

Here is the output:

Array
(
    [c] => 3
    [b] => 2
    [a] => 1
)

No Matches Found:

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

$arr1 = ["m" => 10, "n" => 20];
$arr2 = ["x" => 30, "y" => 40];

$result = array_intersect_ukey($arr1, $arr2, "compareKeysStrict");
print_r($result);

This example shows a case where no keys match. The result is an empty array. It proves the function only keeps keys that exist in all arrays.

Wrapping Up

You learned what array_intersect_ukey does and how to use it.

Here is a quick recap:

• The function compares arrays by keys with a user-defined compare function.
• It returns only keys that exist in all arrays.
• The compare function must return 0, 1, or -1.
• array_intersect works with values. array_intersect_ukey works with keys.
• Use cases differ based on whether you need a value match or a key match.

FAQs

function compareKeys($a, $b) {
    if ($a === $b) return 0;
    return ($a > $b) ? 1 : -1;
}
$array1 = array("a" => "red", "b" => "green", "c" => "blue");
$array2 = array("a" => "black", "c" => "yellow", "d" => "brown");
$result = array_intersect_ukey($array1, $array2, "compareKeys");
print_r($result);

array array_intersect_ukey ( array $array1 , array $array2 [, array $... ], callable $key_compare_func )

function keyCompare($a, $b) {
    return strcmp($a, $b);
}
$users1 = array("101" => "Alice", "102" => "Bob");
$users2 = array("101" => "John", "103" => "Sara");
$result = array_intersect_ukey($users1, $users2, "keyCompare");
print_r($result);

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