<?php
$conn = @mysql_connect("localhost","username","password");
echo "出错了，错误原因是：".$php_errormsg;
?>
返回
Notice: Undefined variable: php_errormsg in /54/885/4Rqx/index.php on line 3出错了，错误原因是：
代码前，加上一行：ini_set('track_errors', 1);