Do you need email integration?
If not, and you're just after a lightweight PHP app that lets everyone manage the calendar through a browser, then the following might fit the bill:
On the other hand if you do need to integrate with email, you will most likely need to use a CalDAV or iCal calendar server.
DAViCal will work with Mozilla, Evolution, Chandler, iCal and iPhone.
Another alternative might be to take a look at some broader CRM packages, as many of these incorporate calendar servers. This would be a wasteful approach if you have no intention of using any of the other features, but if you'd get any mileage out of any other CRM'y stuff (contact management, activity management, issue tracking, time tracking, inventory management, etc) then it could work for you. Sugar CRM seems to be the market leader and is PHP based though I'd recommend taking a look at OpenCRX.