Where $PHP_SELF comes from

Hello everybody,

Can anybody tell me where the value of $PHP_SELF comes from. Most of the times is blank and is causing me troubles in some of my pages. I’m using the next form in my side bar

<form action="<?php echo $PHP_SELF ?>" method="get">
...
</form>

To displays the categories using the function wp_dropdown_categories(), but only works in the main template. When I’m under another template (ie: single post, search results) the form doesn’t work.

Should I change the value of the action parameter to my main page?

Thanks a lot.