Fixed xpath's php:functionString

Added CallerFrame before dom_xpath_ext_function_php's call to xmlXPathEvalExpression in case libxml2 needs to call back into HHVM. 
The xpath query might contain ```php:functionString``` which triggers a call to any arbitrary php function.

Also fixed a bug where the arguments in an xpath php:functionString were reversed. For example, if the xpath was ```/foo[php:functionString('preg_match', '{$regex}', @bar)]```, preg_match was being called as ```preg_match($bar, $regex)``` instead of ```preg_match($regex, $bar)```.