This will work just fine:-
$date = new \DateTime('2013-09-06 10:30pm GMT+1');
$date->setTimezone(new \DateTimeZone('UTC'));
var_dump($date);
See also http://php.net/date for more information on date string formats.
One issue with your date string is the 'GMT+1' part, there is no valid date string for this (hopefully somebody else can help with that), so it is difficult to make \DateTime::createFromFormat() work in this case. However the DateTime constructor seems to cope with it quite well when left to its own devices.