Program flow?

Hi I have a one simple question that I need to sort out before I
continue writing any PHP scripts. Every time I call a script are the
variables reset to the default values? If for example I call action.php
from a form with a hidden value "test", I set $NUMBER to 5. If I then
call action.php again from another form with another hidden value which
means that I do not assign any number to $NUMBER, will $NUMBER now go
back to being its default value or will it stay at 5? So basically I
think my question is, each time one runs a script is it the same as
starting the application all over again? Cheers,
Rory.