$PHP_SELF

I have just installed php. I am new to php. I have read several items(ie. books, online documentation). I have seen references to a variable called $PHP_SELF. I have tried to use it in a form, but to no avail. Here is a line of code where I have used it: <form action=<?php echo $PHP_SELF ?>?form=yes method="post"> However, when I view the souce of this page, the "action" attribute is simply: <form action=?form=yes ..... The $PHP_SELF variable is not referencing anything. Is there something that I have not configured properly? Please help a newbie. Thanks