It's tempting to assume that PHP's $_SERVER array mostly contains fields out of the reach of an attacker, since these are "server" variables. However, that's not always the case; in particular, the seemingly innocuous PHP_SELF field can be a vector for cross-site scripting.

For example, consider the following foo.php:

<form method="POST" action="<?php echo $_SERVER['PHP_SELF'] ?>">
    <!-- ...form elements... -->
</form>

If I visit http://www.example.com/foo.php, $_SERVER['PHP_SELF'] will be /foo.php and everything will work correctly.

But what if I visit http://www.example.com/foo.php/"><script>alert('hello');</script> instead? Then the rendered HTML will be:

<form method="POST" action="/foo.php/"><script>alert('hello');</script>">
    <!-- ...form elements... -->
</form>

This allows injection of arbitrary script running under the host site's context, also known as XSS. Two ways to fix this are:

• Use $_SERVER['SCRIPT_NAME'] instead of $_SERVER['PHP_SELF']. The former is the name of the actual script file and can't normally be manipulated by an attacker.
• Use htmlspecialchars(), which by default will escape double-quotes and prevent a user-supplied string from breaking out of an HTML attribute context.

By the way, this was pretty surprising behavior to me for two reasons:

• The documentation of PHP_SELF is misleading: The first sentence says:

  The filename of the currently executing script, relative to the document root.

  It seems odd that PHP would refer to something like /foo.php/"><script>alert('hello');</script> as a "filename."
• It's pretty bizarre default behavior that PHP will execute /foo.php for a request of /foo.php/bar/baz.