Question

Is it possible when I get a 503 error from a XML stream to not have it stop the PHP script but instead try again?

Answer 1

Your question could benefit from some more context, but assuming you mean the HTTP Status Code 503 you can try with

do {
    $feed = simplexml_load_file('http://example.com/rss.xml');
} while (strpos($http_response_header[0], '503') !== false);

// do something with $feed now

This will repeat the do block as long as the server responded with a 503 Status Code.