OK got it, "callSbin" has to be used here :)
<?php
pm_ApiCli::callSbin('mail_handlers_control', array('--add', '--priority', '50', '--queue=before-queue', '--type', 'global', '--executable', '/opt/psa/admin/htdocs/modules/PMSP/mail-handler.sh', '--enabled', '--name', 'PMSP'))...