The TIOBE index is not usable for web development, simply because it is focused on general-purpose programming languages... which include the four "C" variants with 43% of the share. None of these (okay, C# excepted due to MS ASP) is widely used for web development.
Now PHP ranks second behind Java (though I'd guess if you made a strictly web-focused index, PHP would outrank Java!).
For the "Python/Ruby sucks vs PHP" part:
* the user-base is damn small compared to PHP. Go into any web-development agency and ask if someone uses PHP for web-dev and if someone uses Ruby/Python for web dev. The percentage of shops using PHP will be significantly higher.
* Ruby and Python do not originate from web development, but from "normal" programming. Web capability got "bolted on" onto them with CGI and various frameworks, while PHP originated from the Web and only recently acquired usability as a scripting language.
I brought up the TIOBE index not as an absolute metric but to show that Ruby and Python are not totally obscure languages and have usage similar to PHP. I think your "go into any web-dev shop" test fails for software of interest to developers here who are developing web applications rather than dynamic sites. The web-specific part of PHP is it's ability to act as a template language, inline with HTML. I don't think this is an advantage for web apps as there is often significant logic and infrastructure other than the user interface and frequently several interfaces are desirable (e.g. API, mobile, and web). Mixing together logic and presentation, that is to say the web-specific parts of PHP, make this goal much more difficult.
Ruby and Python were not originally made for web development, but they have been extremely successful and popular in that role. Twitter, Github, Stripe, and Hulu have significant Ruby components, and Instagram, Dropbox, Mixpanel, Pinterest, and Yelp use Python. There are smart people choosing these languages to build their companies with for a reason.
Now PHP ranks second behind Java (though I'd guess if you made a strictly web-focused index, PHP would outrank Java!).
For the "Python/Ruby sucks vs PHP" part:
* the user-base is damn small compared to PHP. Go into any web-development agency and ask if someone uses PHP for web-dev and if someone uses Ruby/Python for web dev. The percentage of shops using PHP will be significantly higher.
* Ruby and Python do not originate from web development, but from "normal" programming. Web capability got "bolted on" onto them with CGI and various frameworks, while PHP originated from the Web and only recently acquired usability as a scripting language.