Anyone know PHP

[jaguar]

I've been doing a few little things in PHP over the last week for a particular purpose and i'm confused about one thing

I've got a script that refers to itself when it runs, checking a value ($id) that would have been set if the page/script is beign viewed for the second time

the code to check that is

if ($id) {

blahblabla

}

Which as i thought checked that $id existed (in the url). It doesn't seem to work. If i get $id to check against a certain value i know it will be it works, but clearly the person who told me this was wrong or i've totally missed something.

Any ideas from the crowd?