1) Some programmers are just either too lazy or too arrogant to read the manual, thus bad programmers. As a senior programmer that has helped numerous juniors in their work, this is probably one of the most common mistakes I see, they write code without checking how the API works, or they ask me why something isn't working when it can be easily found in the manual. This is not specific to PHP and I have seen this in everything from PHP to C++.
2) PHP is dynamic language, similar to JavaScript, types were not something you historically bothered with, thus parsing to int is not something the community has a large shared knowledge about, similar how it is rare in JavaScript. However with the introduction of primitive type hinting it has become more common to do proper parsing, hopefully PHP programmers will learn eventually. Thus it is a thing with dynamic languages, not specific to PHP.
3) Why can't programmers find the filter_var method in the manual and/or in the standard library? This I considered as valid criticism of PHP, the standard library is not structured because of the C like flat structure and many of the stdlib functions are weirdly named, like filter_var, what is filter to begin with?
The solution is to build a better stdlib, not throw out the language. I think the community can build a well designed stdlib if PHP implements operator overloading. The performance issues is no longer valid, PHP 7+ has great performance, thus worrying about losing important cycles is not really true anymore.
PHP as a language works well to build good solutions, I have done so in many projects. The critique here evolves around the stdlib, that is not something new.