I'm trying to do some validation with form data passed to a script and stored in session vars, but not getting very far

I have an array of session var names I want to check to make sure they aren't empty
PHP Code:
$req_vars = array("field1", "field2", "field3"); 
How can I get PHP to check each of those fields ensuring each has a value? This has really got me stumped. I'm using the $_SESSION thing BTW

Thanks