how do i let a php variable change but still be permanent

basically i want to make a site were the user can make their own page in the site. i want it so that all pages are named one by one for example :

page 1

page 2

etc

any ways i decided to have a variable called $num = 1;. and at the end of the code increment by one $num .

$num = 1;
//code here
++$num;

but i have came into a problem $num will always start at one , so how do i change it so that $num is = to the latest increment