How dangerous are you to your ISP if you can run PHP and have ftp access? I mean you could upload pretty harmful scripts and hurt the server if you really wanted to. No? You could, for example, run these accordingly and if the PHP account has enough privilege they will stop the web server.
Unix:
exec('/usr/local/apache/bin/apachectl stop');
Windows:
exec('net stop w3svc');
...or you could get a little fancier than that and give yourself Admin privileges by:
exec('net localgroup "Administrators" username /add');