problem with string param in the query string in php3

I ve the following problem when I pass a string param using the URL string in php3. I call a page with pagename.php3?var=val and I get the parameter value in the other page using: if (isset($var)){ $theval=$var; } defined a function included in an external misc include file. When I use this with a numerical value all its ok, but when I use this with a string parameter this doesn´t work. why? Is there any standar method for track user session variables other than pass it in the URL string??.