Substract one array from another

the are two built-in functions to do something similar: array_diff and array_diff_assoc - but both won't work in your case.

so, to do what you want, you'll have to change the markup of your $a3 a bit to fit these functions (take a look at the documentation), or you'll have to loop $a3 and delete the elements from $a2 manually like this:

foreach($a3 as $k){
    unset($a2[$k]);
}

Don't get it.

$aNew = $a2;
foreach($a3 as $key) unset($aNew[$key]);

you need this one?

This can be done with php built-in functions.

Since the array $a2 contains the values of the keys you want to remove, you need first to create an array containing the values of $a2 as keys using array_flip. Then you can just use array_diff_key. So try this:

$aNew = array_diff_key($a2, array_flip($a3));

Note that you need php > 5.1.0 for this to work.

$aNew = array();
foreach (array_diff(array_keys($a2), $a3) : $key) {
   $aNew[key] = $a2[key];
}

This sould do the job. Grap the keys from a2 remove all keys which exist in a3 and then copy the remaining keys to a new array. You could also remove the keys from the old array. This is up to you.

Edit: +$

The difficulty here lies in that you compare key of one array with values of another one. E.g. by the use of array_diff_ukey. Excerpt from PHP manual:

  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) }

Your second array is not associative, so this would compare "a" with 0, e.g. So it's:

$compareFunc = function($key) uses ($a3) {
 if(array_key_exists($key, $a3) {
    return true;
 }
 else return false;
}

array_map($compareFunc, $a2);

can't be used neither, because this function is used to modify but not to reduce the array.

$a4 = array();
$compareFunc = function($key) uses ($a3, &$a4) {
 if(array_key_exists($key, $a3) {
    array_push($a4);
 }
} 

would do the job.
What I am trying to achieve is: Play around with the PHP - functions that provide callbacks to you when the standart - functions don't succeed. Try using the concept of binding, when the describtion of the callback doesn't fit your needs. With PHP's Lambda - functions you can bind objects / variables to a function, so that you have access to variables that are not passed to the callback. Get creative!!!

Try using array_diff but you need to make some changes on your $a3.

$a3 = array('bat', 'Eagle');
//and then
$a4 = array_diff($a2, $3);

because without specifying a key (in key => value format), the string will automatically become a value instead.