Ye' old general discussion board. Basically, for everything that isn't covered elsewhere. Come here to shoot the breeze, shoot your mouth off, or whatever suits your fancy. This forum is not for asking programming related questions.
That specific code gives you the ability to say 'use this script when posting the form' in an abstract way; you don't have to hard-code the script name into the form. It's useful when you're including a script with a form on it, etc.
It's part of the $_SERVER superglobal, and in fact, using $PHP_SELF indicates register_globals = On, which is a bad thing. Use $_SERVER['PHP_SELF'] instead.
Before you contemplate using it, however, read the results in Google search: 'php PHP_SELF insecure'.
If i remember well, last we tried it (overhere @phpdevnet) some browsers didn't seem to handle '' as target very well. It was then that we found that '#' was a better choice
It would have been better if i remembered which browsers (and which versions) had problems with the '' target... This way i could regularly check up on them... Now it seems as if this is going to become a 'rule', but nobody remembers where it came from (Which is not a good thing!)