Call CSS with a PHP function

Hello everyone,

I have a code that enables me to change my CSS for every new visit.
Here it is:

[code moderated per forum rules - please read http://codex.wordpress.org/Forum_Welcome#Posting_Code for posting code]

When I use it on a regular php page, I call my CSS using this <link rel="stylesheet" type="text/css" media="screen,projection" href="<?php echo $themecss; ?>">

However, when I want to use it in WordPress, this doesn't work. I guess I should use a WordPress funtion to call it, but I can't find how to do it.
Anyone can help me on this?
I hope my English is okay and you understand what I mean....