Create a parser targeting the host PHP version, that is the PHP version we're currently running on. This parser will not use any token emulation.

Create a parser targeting the newest version supported by this library. Code for older versions will be accepted if there have been no relevant backwards-compatibility breaks in PHP.

Create a parser targeting the given version on a best-effort basis. The parser will generally accept code for the newest supported version, but will try to accommodate code that becomes invalid in newer versions or changes in interpretation.