For me, I started using Python because I could write the front end (web-facing) and back end code in the same language.

Meaning I can write back end services like mail server and share models and other code across both.

You could argue that you can write back end services in PHP, however, it was no designed with that intention and in my honest opinion it does not translate very well because the infrastructure in terms of open source libraries just don't exist as PHP is primarily a web language.