The good parts of PHP, just off the top of my head: Extremely forgiving syntax which makes it quick and easy to bash together something that just works, implicit type conversion, a vast array of built-in functions, readability, near-ubiquity, generally very strong documentation with a healthy focus on example code, and the way it interpolates with static HTML, which I'm sure there's a word for but I can't remember offhand.
What I see as the big strength is how easy it is to set up mod_php - possibly in no small part due to how most Linux distributions have solid defaults for mod_php5 and keep it up to date. The things you listed I'm not so sure about.
I don't think it's the syntax that's forgiving as much as the type coercion. JavaScript is actually more forgiving syntax wise, for example - PHP throws a fatal error when a semicolon is missing at the end of a line, JS interpreters fill it in. Unlike PHP, though it will throw a ReferenceError and die upon referencing any undefined value. PHP just coerces that to something falsey... they'll both die trying to call undefined functions or methods.
I don't see how PHP is not more readable than Python or Ruby, either. There's really nothing about the language itself that makes it readable or better for beginners.
The docs are definitely good, I agree. I think they need to do something about the outdated and not-so-well curated comments on each entry though.
Near ubiquity is good, but it also makes a lot of good PHP resources get lost in horrible noise. WordPress, for instance - if you search to solve anything, you find scores of outdated tutorials offering solutions to problems which have been solved by the official branch for years. Then, you find more blogs blog-spamming the incorrect solution... so it's actually easier to search for and find Rails or Django help, in my experience. A large percentage of the PHP code out there is notably terrible, for whatever reasons.
The 'interpolation' would be called templating. You can do this in most languages (erb for Ruby, for example) but it's not always considered a good idea to give the template language logic capabilities. It happens that this is the default for how PHP and Apache work together, and I think the straightforward concepts of files, directories, scripts it's easier for beginners to understand and work with than framework style URL routing.
The large software eco-system centered around tools consumers use such as WordPress or joomla has helped PHP grow.
Set up is easy, but the majority of PHP users probably never set up their own Linux boxes with Apache anyhow. If they wanted to, though it would be easier than say, Django since last I checked Ubuntu was still providing Django .96 packages.
The ease of setup comes from every shared host offering PHP and MySQL. If they all offered Python, Django and Postgres support, with cpanel configs and PhpPgAdmin or the like, people would magically find Django really easy to set up. That's just how it goes when you have lots of market share.
Date and time handling with date() and strtotime(), magic methods, simple and practical array handling, proximity to the HTTP so you can solve problems the way you want instead of the way someone tells you you should.
