As supplied, PHx plugin sets up the path to the main include file using
$modx->config['rb_base_dir'], which I think is from the Tools > Config > Interface & Features > File base path setting.
(From Manage Elements > Plugins > PHx):
include_once $modx->config['rb_base_dir'] . "plugins/phx/phx.parser.class.inc.php";
When you move servers, you need to update that setting by clicking the Reset button next to it (you'll also have problems finding your image directories etc. if you fail to do this). Also, you should update the File Manager Path at the top of the Configuration's File Manager tab.
I prefer to change the include line in PHx to use the
base_dir setting:
include_once $modx->config['base_dir'] . "assets/plugins/phx/phx.parser.class.inc.php";
:) KP