Adding an existing script into a wordpress plugin

Hello all

I am trying to add a PHP script I have into a plugin.

the problem is it uses variables in the URL

for example if I include my page using a shortcode it needs to receive variables from url like

http://sitename.com/pagename?contact=yes

the plugin would then need to grab the variable

something like

$contact=$_POST["contact"];

can this work in wordpress?

I have been told that wordpress blocks url variables

what would be the best way to pass these variables to my plugin?

If there is a simpler way to pass these variables please include an example

Thanks for your thoughts and advise on this

Paul