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.
You are not answering the question that was asked. ianl's post was directly in reference to web application development, not systems programming. Of course you wouldn't do that with PHP. That's like saying "yeah, it's a great knife, but it can't change my oil." Well...sure. Nobody's saying it can. But it does what we're actually talking about just fine, and I am skeptical of ianl's claims that there are missing pieces of the infrastructure for web development.
(BTW: Puppet not playing nicely with PHP is new to me as I use it regularly. And I don't think you get much with Mongrel that nginx/FPM don't do well enough.)
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.