如何在代码中直接更改php变量的值？

I would like to change the value of assigned php variable. For eg: consider a variable as $a = "yes"; I like to change this variable value as $a = "no"; directly in the code by performing some action in another page. Is it possible to achieve what i expect????

make a session in the first page like this:

$_SESSION['a']="yes";

make the second page work like this:

secondpage.php?a=no

so you used the get method to pass the variable ... and in the code:

<?php if(isset($_GET['a'])) $_SESSION['a']=$_GET['a'];?>