Hi,
I've had a look through the spec and I'm confident I can turn this around quickly for you.
I am a UK based PHP web developer, with over 8 years of industry experience - working on everything from large-scaled back end systems to mobile apps, facebook apps, wordpress plugins and more.
I would be making heavy use of ajax to make the individual chat requests as quick and light as possible, to ensure the whole system feels responsive and easy to use.
I hope to work with you very soon.
Kind Regards,
Liam