pht requires a ZTS (Zend Thread Safety) version of PHP. Currently, only PHP 7.2 is supported. Support for PHP 7.3 (which will be released at the end of 2018) will be coming soon. PHP 7.0 and 7.1 are unsafe to support, due to race conditions in ZTS mode.
pht should be compatible for any OS with the pthread library (or pthread-win32 library, which is for Windows).