Searching for it I found this blog post: https://www.npopov.com/2017/04/14/PHP-7-Virtual-machine.html
Does it cover "everything" needed to add a new opcode, or all the places I'd need to touch in the engine? Or is it better to just start grepping in the code-base? Maybe there's a commit that can be used as a prototype or example?
Edit, last opcode added was this: https://github.com/php/php-src/pull/7019/files#diff-773bdb31563a0f907c75068675f6056b25f003e61f46928a31d9837ae107460d
PHP source code is written in the c programming language. That is what you are looking at in your link to the "last opcode added". That's not opcode it is source code. The output of the PHP interpreter (Zend Virtual Machine) reads the contents of php files and turns it into opcode. Opcode is not written directly by programmers.
If you want to write a custom extension for PHP you will need to write it in c and structure it so it can be loaded as an external mod. To say, "It's not trivial" would be a massive understatement.
There is a hybrid system that allows you to write PHP-like code that can be compiled into a standalone PHP extension. Check out the Zephir language. Stack overflow has lots of questions tagged with 'zephir'. I have read about it but not used it.