Go for Python, here is why: PHP is a nice enough language that can do almost every webapp task, however, you will type syntax far more than logic and will shoot yourself in the foot more often then not. Python is a clear and concise language compared to PHP, libraries and modules are often of a higher quality than PHP and can often be understood if you choose to go and look at their code. For developers with PHP experience, PHP is ofcourse easier, I have experience with both languages and have a clear prefference for python due to it's readability.

If you just want to do a small webapp, Flask is awesome, for a larger more complete framework you should go with Django.