Filtering a PHP array by key instead of value.

array_filter_key.php

<?php

/**
 * Filtering a array by its keys using a callback.
 * 
 * @param $array array The array to filter
 * @param $callback Callback The filter callback, that will get the key as first argument.
 * 
 * @return array The remaining key => value combinations from $array.
 */
function array_filter_key(array $array, $callback)
{
	$matchedKeys = array_filter(array_keys($array), $callback);

	return array_intersect_key($array, array_flip($matchedKeys));
}