Can anyone recommend a good Helpdesk PHP/Mysql solution?
what features do you need?
zendesk.com is an option that is nice if you don't mind the monthly bill.
or build your own, based on FAQ package, discussion, calendar, etc
If you are managing an IT helpdesk, I would recommend you to go for GLPI
If it is for a helpdesk oriented toward supporting a software, several tracking solutions exists (Trac, Redmine, etc…) although not all of them are PHP (the latter was not),
If it is a CRM helpdesk you are looking for osTickets is pretty solid, (but Hesk or eTickets are not bad either),
now, if you would be more specific maybe we could point you to something better?