Example:
source:
$var=X
url call:
....?changeSourceTo:Y
php:
//do something? (what)
source:
$var=Y
This should be useful for a rapid change of the source code without opening it and without passing the next call the same parameter to the url.... (I know I could use a file with the var parameter and if $_GET['var'] change the file, or maybe better set up a database...)