The main reason is that it's the least-competitive language to hire for in the Bay Area. There are other reasons too, though.

Laravel is easily competitive with Rails and Django. PHP7 is usually faster than Python and Ruby in most cases (it's roughly surpassed Facebook's HHVM). Composer and the PHP ecosystem give you very wide and convenient access to high-quality vendor libraries for just about anything you need (similar to gems/npm).

It's not as if it's broken. You can write a fantastic web application or web service using PHP. You can make a mess out of it for sure, but I've seen plenty of Rails projects turn into a big mess too.

Professionally, I work full time as .NET architect, but for my own projects, I always use PHP.

1) It just works. 2) Plenty of great frameworks. I especially love Laravel, which has _everything_ from a great IoC to ORM, as well as a huge community around it. 3) It's so easy and quick to get started. 4) HHVM (and also PHP 7) just makes PHP fly.

For me it's mainly because of Laravel. Everything just works as I would expect, to a level I can often figure out how to use internals without consulting the documentation. With that said I mostly have experience with PHP so perhaps there are superior alternatives in other languages.

One would be that PHP is all you know, another might be "wordpress".

Large community. With recent updates its fast and has modern features. A lot of PHP developers out there so hiring is not an issue (you just need to filter out the bad ones - and there's a lot of those too).

If your project is primarily web based its a really good choice.

At the end of the day though PHP is just a tool and what matters is how you use it and what's the outcome. If you are familiar with it then why not use it?

I personally feel like the biggest shortcoming of PHP is its legacy and the huge amount of bad code that you can find by searching on Google.

Hiring pool (especially if you're aiming for low cost remote contractors)