Indeed, jjanvier has right. We can not put `chmod -R 777` in the documentation, it is not very safety... We can just put a link to the symfony installation doc... 

Finally I think that `chmod -R 777` will not work because if you run a sf command with a local (different than apache), the log and cache files will get the rights defined with umask property, no? By default, I think it is 755, no? those generated files get the user and the group of the user who run the command. For example local user clear the cache, all cache files will have the local user as owner (and his default). After that, apache will can not overwrite cache files and it will return you a exception!

I hope you understand me...