How much PHP should one know BEFORE using CodeIgniter?

[eluser]JPrieto[/eluser]
i want to create am "online social community" -- similar to ELGG
but with a more functional design

ELGG requires even the home page to be modified (custom_index mod)
and has weird SERVER REQUIREMENTS

so i want to create one that will work in ANY server that has MySQL & PHP
and will work well straight out of the box

and make it freeware and open source

and that if what i want to do