Opensource Alternative to Share point

Hello all,
I'm almost being compelled to implement Microsoft Share point Server.
However I have some reservations cause I'm traditionally a PHP/MYSQL LINUX Developer.
Please If you've used Microsoft SharePoint before;
what exactly are the features you used ?
Is there an open source alternative you've seen heard of or used before ?
Your response will be appreciated.

Please make this popular by digging so more people can see it and help if possible.

Thanks.
Prosper.