如果没有定义变量，这个php形式如何工作？

Question

how does this php form work without defining $SCRIPT_NAME variable ?

<form action="<?php echo $SCRIPT_NAME ?>" method="post">

Answer 1

This code relies on the ancient, deprecated and horrible register_globals feature which creates global variables from all the $_REQUEST, $_COOKIE and $_SERVER fields.

I'd highly suggest you to get rid of this code and disable the register_globals setting.