Here is some code to take the difference of two arrays. It allows custom modifications like prefixing with a certain string (as shown) or custom compare functions.

<?php 
 // returns all elements in $all which are not in $used in O(n log n) time.
  // elements from $all are prefixed with $prefix_all.
  // elements from $used are prefixed with $prefix_used.
  function filter_unused( $all, $used, $prefix_all = "", $prefix_used = "" ) {
      $unused = array();

      // prefixes are not needed for sorting
      sort( $all );
      sort( $used );

      $a = 0;
      $u = 0;

      $maxa = sizeof($all)-1;
      $maxu = sizeof($used)-1;

      while( true ) {
          if( $a > $maxa ) {
              // done; rest of $used isn't in $all
              break;
          }
          if( $u > $maxu ) {
              // rest of $all is unused
              for( ; $a <= $maxa; $a++ ) {
                  $unused[] = $all[$a];
              }
              break;
          }

          if( $prefix_all.$all[$a] > $prefix_used.$used[$u] ) {
              // $used[$u] isn't in $all?
              $u++;
              continue;
          }

          if( $prefix_all.$all[$a] == $prefix_used.$used[$u] ) {
              // $all[$a] is used
              $a++;
              $u++;
              continue;
          }

          $unused[] = $all[$a];

          $a++;
      }

      return $unused;
  }
?>