Kinda silly question

Maybe this one is a silly question ... Could someone teach me about that damn $PHP_SELF ??? coz until now i can't get it work in anyway .. so i have to make a lot of pages. i mean like whenever i wanna use it, it gaves me an error ... ex: if ($comp) { bla bla bla } else { echo "<form name='form' action='$PHP_SELF">"; echo "<input type='text' name='comp'>"; echo "<input type='submit' value='get it'>"; echo "</form>"; or echo "<form name='form' action='$PHP_SELF">"; echo "<input type='text' name='comp'>"; echo "<input type='submit' value='get it'>"; echo "</form>"; print "$comp"; It always said that : Warning: Undefined variable: comp in c:\homepage\modify.php on line 10