- array_reverse: documentation ( source)
- implode: documentation ( source)
- array_walk: documentation ( source)
<?php
$arr = array('a', 'b', 'c', 'd', 'e', 'f');
$str = implode(array_walk(array_reverse($arr), function($v,&$k){if($k%3==0)$v.=';';}));
echo $str;