Create php function for tracking the url parameter once present then save the parameter

i have a website . www.website.com . And there is a page called functions.php , in which i can write evry function

if some one visited the website with the key parameter in url i have to use their key parameter value in registration form

so

if the link is www.website.com/registration?php/?key=123

so in the register form i can create a hidden filed called user_key like

<form method="post" action="">

other fields 

<input type="hidden" value="<?php echo $_REQUEST['key']?>" name="user_key" /> 


</form>

here everything is working fine .

but what happen is if some one come through

www.website.com/page?php/?key=123 and then he come to registration?php . Then how can i get the key parameter value ?

that means may be he come different page of my website with ?key=value . But once he came to my website with this value then i have to get this value until he close the browser .

I can't use session for that

Or

Can i make a global variable . If some one have ?key parameter in url in function.php i will save the value of key in a variable and that variable value will become none when browser closed or some one without key is coming to website ?

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