Web based Online Billing & Project Tracking Application

I am looking at buliding an online billing and Project tracking system. I run my online business where I hire freelancers. Currently I track all my data on google docs and an excel spreadsheet, however with business growing it doesnt seem to be a very robust system from a long term perspective. What I am looking for is an online billing and project tracking system. Let me know what back-end do you think we should go for ? Whether, PHP. SQL or anything else. I am looking for a very secured system in place. The logic of this online application is very simple... I get the project from a client, I assign it to a freelancer, the freelancer is able to see the project details under his id (limited information). Project is completed and sent back and I update my records. I am then able to send an invoice to the client from this system. I also pay the freelancer for the work he did. So the application needs to have the login ids where the freelancer becomes the master in his own account when he is hiring freelancers to work for him while I become master in my own account when I am hiring freelancers to work for me. The project needs to be completed in phases / separate modules linked together. It also needs to have a very very professional and attractive frontend. Please do get back with a proposal on the same and especially what backend you propose to use for the same.