$q = $_GET['q'];
// Load and parse the XML document
$rss = simplexml_load_file('http://search.twitter.com/search.atom?lang=en&q=$q&rpp=100&page=1');
This returns results containing "$q" instead of results containing the value of $q. Why won’t it work?
Change your quotes to double quotes in the simplexml_load_file line
PHP will automatically resolve the variable only if the string is in double quotes.