アルゴリズムについてです、助けてください

$arr = array(<br />  array( "A",  "B",  "C"),<br />  array("1",  "2",  "3", "4"),<br />  array("X",  "Y",  "Z"),<br />);<br /> <br />function foo($d) {<br />  $r = array_pop($d);<br />  while($d) {<br />    $t = array();<br />    foreach(array_pop($d) as $k1=>$x) {<br />      foreach($r as $k2=>$y) $t[] = "$x $y";<br />    }<br />    $r = $t;<br />  }<br />  return $r;<br />}<br />print_r(foo($arr));<br />

 21] => B 4 X<br> [22] => B 4 Z<br> [25] =>C1Y<br>    [26] => C 1 Z<br> [27] => C 2 X<br> [28] => C 2 Y<br> [29] => C 2 Z<br> [30] => C 3 X<br> [31] => C 3 Y<br> [32] => C 3 Z<br> [33] => C 4 X<div class="clear"></div>