php如何判断字符串是否存在

<?php 
if(stripos($sql, "insert") === 0){ 
 $excuteResult = mysql_query($sql); 
 if($excuteResult){ 
     $sql = $sql2."where id = (select max(id) from subway2)"; 
 } 
    } else if(stripos($sql, "delete") === 0 || stripos($sql, "update") === 0){ 
 
if (strpos($foo, “my”)===0) { echo(‘I find!’);}