How to Create a PHP Function for Key-Based Array Intersection Using Custom Callbacks Without Built-In Functions

The array_intersect_ukey function in PHP compares the keys of two or more arrays using a custom callback function and returns the elements from the first array whose keys are present in all arrays. Below is a custom implementation of this functionality without using PHP’s built-in array functions.

Code Example:

<?php
function custom_array_intersect_ukey($array1, $array2, $keyCompareFunc) {
    $result = [];

    // Loop through the first array
    foreach ($array1 as $key1 => $value1) {
        // Check each key in the second array
        foreach ($array2 as $key2 => $value2) {
            // Use the custom callback function for key comparison
            if (call_user_func($keyCompareFunc, $key1, $key2) === 0) {
                $result[$key1] = $value1;
                break; // Move to the next element in the outer loop
            }
        }
    }

    return $result;
}

// Example usage
$array1 = ['a' => 1, 'b' => 2, 'c' => 3];
$array2 = ['A' => 10, 'c' => 30, 'd' => 40];

$keyCompareFunc = function ($key1, $key2) {
    return strcasecmp($key1, $key2); // Case-insensitive comparison
};

$commonKeys = custom_array_intersect_ukey($array1, $array2, $keyCompareFunc);
print_r($commonKeys);
?>

Output:

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

Explanation:

1. Input Parameters:
   • $array1: The first array to compare.
   • $array2: The second array to compare.
   • $keyCompareFunc: A custom callback function to compare keys.
2. Result Initialization:
   • Create an empty $result array to store elements with matching keys.
3. Outer Loop:
   • Iterate through the keys of $array1.
4. Inner Loop:
   • For each key in $array1, check all keys in $array2.
5. Custom Callback:
   • Use call_user_func to invoke the $keyCompareFunc for key comparison.
   • If the custom function returns 0, add the key-value pair from $array1 to $result.
6. Output:
   • Return the $result array containing elements with matching keys.

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