Most companies and jobs have a single focus such as; Java, Python, Ruby, C family, PHP, etc. Therefore, employment as a python developer would not require knowledge of PHP. There would be some jobs that perhaps ask for both though, but that is the same with all opportunities here and there. Does it help to know more than a single language, of course, can you do well with a specific focus yes. If you dsire to wrk at a Python shop than Python is the way to go. That will limit your focus on jobs, but that can be good and bad depending on how you look at it.