PHP Permutation 3个数组中的3个元素，具有独特的组合

Question

Can somenone help me to find what is wrong in this code where it will generate only 3 element combinations instead of 4.

 function pc_permute($items, $perms = array( )) {
    $back = array();
   if (empty($items)) {
     $back[] = join(' ', $perms);
    } else {
  for ($i = count($items) - 1; $i >= 0; --$i) {
        $newitems = $items;
        $newperms = $perms;
        list($foo) = array_splice($newitems, $i, 1);
        array_unshift($newperms, $foo);
        $back = array_merge($back, pc_permute($newitems, $newperms));
    }  
 }
return $back;


}
var_dump(pc_permute(array('123','124','312','555')));

this code output combinations of this 4 elements of array.(e.g. ('123',312','124','555') What I need is to output combinations of 3 elements from this 4 (e.g. ('123','312','124') ('123','124,'555') ..... etc

Please don't mark this as duplicate as the other thread has only a permutation but didn't satisfy my conditions.

PHP or Javascript code will do. Thanks!