<?php 
$str="select ziduan1,ziduan2,ziduan3 from";
preg_match("/select\s+(.*)\s*from/",$str,$matchs);
$arr = explode(",",trim($matchs[1]));
print_r($arr);//Array ( [0] => ziduan1 [1] => ziduan2 [2] => ziduan3 ) 
?>