PHP Based Email Help Desk System

I am looking at developing a php based helpdesk email system... My requirement is given below.. I get a lot of sales queries emails from clients which I respond to using canned / standard replies or customized based on client mail. We are 5 working in the team and thus it is important that we don't work on each other's emails. At the same time we should be able to follow up on the emails received from clients after a few days if we dont receive a response from them. Basic Features Needed ==================== Filter of emails based on certain criteria automatically Merging of duplicate / related emails Option to choose which email I want to use for sending the mail...For example supported related issue I will use , sales related will go from .... Assigning of tickets to another department / User Closing of tickets Auto Opening of Ticket when a response comes and goes back on top of queue. There are open source programs available or you can build from scratch as well. Building from scratch is preferred as then it can be customized per need. I need a very professional GUI. Let me know of your best price and turnaround time for the same,....