What is the significance of using $_SERVER['PHP_SELF'] instead of $_PHP_SELF in PHP form actions?

Using $_SERVER['PHP_SELF'] instead of $_PHP_SELF in PHP form actions is significant because $_SERVER['PHP_SELF'] is a predefined variable in PHP that contains the filename of the currently executing script. On the other hand, $_PHP_SELF is not a predefined variable and will throw an undefined variable error. Therefore, using $_SERVER['PHP_SELF'] ensures that the form action points to the current script, which is a common practice to prevent security vulnerabilities such as cross-site scripting (XSS) attacks.

<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post">
  <!-- Form fields go here -->
</form>