Is PHP worth learning in 2021

Depends on what you want to build with Php. I'm a software engineer I've worked with many languages in the past and Php is one of them. It's fun to work with it.
In my opinion, it worths learning PHP in 2021. Don't forget that 70% of the websites is built with php. It has a huge community, frameworks and tools.

PHP is having very good community and also, with every new version releasing is itself witnesses it's continuous improvement.

Anyway in stackoverflow survey 2019, python was voted above php. But this doesn't mean php is lesser than python. Languages jump over each other.... Just like once php was choice now python.
Also, there is difference in python, php and javascript.
stackshare.io/stackups/javascript-...

One of the reason why python and javascript is getting popular because they are suggested for ML to students. However, I will also request you to look FFI which is opening new avenues for ML with php 7.4+.

zend.com/blog/php-foreign-function...

You can't ignore any language. Atleast, we need a basic knowledge of creating crud with php, nodejs and python. The next master language you can choose

Just PHP ? No clearly not.
However, there are many frameworks such as Symfony and Laravel built on top of PHP and in my opinion these are worth learning, they are widely used, easy to learn but still very powerful.