which is better for my project: Django, Plone, php, or ruby on rails
I think you should stick with PHP : 

you already know the language
you already have the tools for development
you already have a code base

I think you should see how your site behaves in production and see what are the most wanted user needs before spending time rewriting everything, and during this time not beeing able to answer correctly to their needs.
And while your site sends its first bits, it will give you a lot more info on what to expect from the version 2, from a technical point of view.
Edit : I am not pro-php, I just think you should stick whit the language you already know, until your business gives you time to learn something else.
I have a php video hosting site, not a typical video hosting site, but i think you put it in that category.
I'm almost done with it, I'll launch it maybe next week, i created it in php because my partner wanted to get it done so fast, and the fastest way for me to do it was php, because i know it could be done with php mysql, ffmpeg and ffmpeg-php and some other multimedia packages. I don't know what it takes to do it in other languages.
now I want to launch the site, and re-write another better version, because i don't like the current version, my partner likes it but he's on the business side and I'm on the development side lol, so i decide :D
I don't know much about the php frameworks, I've seen them all, but I don't think they are that good for such websites
I was thinking maybe Django, but Django is created mainly for CMS websites, I don't know if i can use it for my site
I've never used plone but i think it's good for my site, I don't have experience with it so I would like to know what you think
Ruby on rails seems to be another option but I've never seen any video hosting site using ROR
So what is the best language to re-write my site? or should i stick with php?
edit
i already know python, so it's not hard for me to switch to python framwork if it's better for such project...I have some ROR knowledge...and I'm gonna use php for now as you said guys but I'm talking about the future
I don't want to make this question an argumentative question, because it will be closed, so let ask another question, can i do such website with django because i know django, but i have never used it for such thing, i used it for cms, i don't know if it supports ffmpeg, multimedia and such things
sorry for posting an argumentative topic, i hope my question is now better :D