The basics have already been answered here. But is there a pre-built PHP lib doing the same as Lingua::Identify from CPAN?
There's a PEAR package Text_LanguageDetect that I've used before. Get's the job done well enough. I'm not sure of any other libs that are more mature.
1- You could do it yourself (the hard way) - detecting both language and codepage by looking at character and n-gram frequencies. You would need lots of "training" data, but it's doable.
2- You could run a perl script to do the detection for you(much easier).