$re = '/\&([\w-_]*)=([\w\d\.\,-_]*)/m';
$str = 'devices/VK_GATEWAY/messages/devicebound/%24.to=%2Fdevices%2FVK_GATEWAY%2Fmessages%2FdeviceBound&%24.ct=application%2Fjson&%24.ce=utf-8&p2=3.3&p1=12&p3=4.4&messageId=6c519ec3-6d60-463c-9a47-2159a72d91f9';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php