Use string UTCDateTime argument for 32 and 64-bit compatibility

Author: jmikola

tests/bson/bug0631.phpt

@@ -5,7 +5,7 @@ PHPC-631: UTCDateTime::toDateTime() may return object that cannot be serialized
 
 require_once __DIR__ . '/../utils/tools.php';
 
-$utcdatetime = new MongoDB\BSON\UTCDateTime(1466540755123);
+$utcdatetime = new MongoDB\BSON\UTCDateTime('1466540755123');
 $datetime = $utcdatetime->toDateTime();
 $s = serialize($datetime);