The "array_diff_ukey()" function is used to compute the difference of arrays using a callback function based on the keys of the arrays used in comparison.

    array_diff_ukey(array1,array2,array3...,function)

In the above syntax "array1" is the array to be compared with, "array2" is the array to be compared, atleast two arrays should be there, "array 3" is optional. Comparison is done using the "function" specified.

    <?php
    function key_compare_func($key1, $key2)
     {
      if ($key1 == $key2)
      return 0;
      else if ($key1 > $key2)
      return 1;
      else
      return -1;
     }
    $array1 = array('blue'=> 1,'red'=> 2,'green'=> 3,'purple'=> 4);
    $array2 = array('green'=> 5,'blue'=> 6,'yellow'=> 7,'cyan'=> 8);
    var_dump(array_diff_ukey($array1, $array2, 'key_compare_func'));
    ?>

    array(2) 
     {
      ["red"]=>
      int(2)
      ["purple"]=>
      int(4)
     }

In the above example the "array_diff_ukey" checks for keys in two arrays "array1", "array2" using the function "key_compare_func" based on the keys to display the elements that are not common in both arrays, but present in "array1".