What are the potential pitfalls of not defining variables like $PHP_SELF in PHP scripts?

Not defining variables like $PHP_SELF in PHP scripts can lead to security vulnerabilities such as cross-site scripting attacks. It is important to always define variables to prevent any user input from being directly executed as code. To solve this issue, you can define the $PHP_SELF variable using $_SERVER['PHP_SELF'] to ensure that it contains the correct script name.

$PHP_SELF = $_SERVER['PHP_SELF'];