PHP配列の組み合わせ

<?php<br />$set = array('a','b','c','d');<br />$len = count($set);<br />$res = array();<br />for($i=0,$k=$len;$i<$len;$i++){<br />  $res[$i] = $set[$i];<br />  for($j=$i+1;$j<$len;$j++){<br />    $res[$k++] = $set[$i].$set[$j];<br />  }<br />}<br />ksort($res);<br />var_dump($res);<br />?>

array (size=10)<br />  0 => string 'a' (length=1)<br />  1 => string 'b' (length=1)<br />  2 => string 'c' (length=1)<br />  3 => string 'd' (length=1)<br />  4 => string 'ab' (length=2)<br />  5 => string 'ac' (length=2)<br />  6 => string 'ad' (length=2)<br />  7 => string 'bc' (length=2)<br />  8 => string 'bd' (length=2)<br />  9 => string 'cd' (length=2)<br />