Since in_array() doesn't work for multidimensional arrays, this recursive in_array_r() does the trick

in_array_r.php

<?php

/* Check if an item $search_item is in the multidimensional array $items */

function in_array_r( $search_item, $items, $strict = false ) {
  foreach ( $items as $item ) {
    if ( ( $strict ? $item === $search_item : $item == $search_item ) || ( is_array( $item ) && in_array_r( $search_item, $item, $strict ) ) ) {
      return true;
    }
  }
  return false;
}

/* Usage exmaple */
// $cars = array( array( "Volvo", 22 ), array( "BMW", 15 ), array( "Saab", 5 ), array( "Land Rover", 17) );
// to check if BMW is in $cars:
// echo in_array_r( "BMW", $cars ) ? 'found' : 'not found';
?>