…no template system. There’s PHP itself, but nothing that acts as a big interpolator rather than a program.
As if that was a bug and not a feature. PHP succeeds because there is one PHP templating syntax and it is called "PHP".
PHP started life as a templating language, and that's where its soul lies. Which is important, because if a non-programmer wants to start editing dynamic HTML pages, most likely the first thing they open in their text editor will be a template. And if a customer wants to change the website, but doesn't want to pay a highly-trained programmer (we're very expensive, especially because none of us really wants to be paid to edit templates all day), they'll hire someone who knows how to edit the theme. For every seasoned programmer in the world, there are half a dozen themers who know how to edit HTML and CSS, and maybe how to embed a "foreach" loop that calls a stock WordPress function, or edit an argument list, or swap one Drupal function name for another. And for every themer there's a dozen non-programmers who dream of knowing enough to be themers.
This class of user is what every other platform tends to neglect. Ruby and Python programmers tend to design tools that are intuitive to other programmers. We build "frameworks", where one must tinker and monkey-patch and fork to such a degree that only another programmer can download and assemble a working system, let alone customize it. (One word: Gemfiles.) We write programmer-centric documentation filled with jargon terms from Javaschool, like "object". (PHP themers often do not understand OO, and often don't need to. This, all by itself, could be why PHP is so popular.) But one leaves a lot of money on the table when one designs software systems exclusively for users whose programming education entitles them to six-figure salaries. There aren't enough such people on earth; that's why they earn six-figure salaries.
Designing the minimum viable product for borderline-non-programmers who want to tinker with dynamic web pages is hard work, though. PHP evolved into that niche, but if you don't start there it's hard to steer there deliberately. For me, trying to remember when object inheritance and version control and function definitions were mysterious is like trying to remember when I didn't understand algebra. (And, believe me, I really didn't understand, once. But I'll be damned if I can remember why not.) It's so hard to keep one's eye on that ball that even the PHP developers lost the plot long ago: PHP has long since bulked up with features borrowed from other languages. But if you ignore its newer features the soul of PHP is still a little templating language that you can edit into the HTML on your server; change a filename extension and reload and you're good to go. And, the thing is: You can write a crazy-complicated core system in PHP, but then expose it as an API at the template layer, and those tinkerers hacking at the HTML on their servers will be able to use it.