For something like a personal recommendation system, machine learning type of stuff on a website, what language would be best?
I recently did some research into this for a project at my day job. It was for a recommendation system and the options were php,perl or python.
PHP was out almost immediately, there were no good 3rd party open source libraries and the language itself is not as well suited to any kind of complex real programming.
Python had a few libraries that i wanted to try out and Perl didn't, so I went with Python. In the end, none of those libraries were useful to me but besides library support I prefer python personally anyway so that was the right decision.
Because your question is very vague I can only suggest that you don't use PHP and select the language based on library support for your specific problem area and your comfort in that language. I would say that library support is the biggest factor in your decision and language familiarity/preference is a close second.
Interested in Collective Programming for the web -- Ruby or Python or PHP?