Why is not 25? Strange behavior in 4 cases

Question

https://code.sololearn.com/w17k1z0OOy5S/?ref=app

Answer 1

Strange, doing preinc returns 16 🤔

Answer 2

The order of which the post increment is evaluated is undefined https://stackoverflow.com/questions/47766470/why-these-both-post-increment-in-php-gives-the-same-answer

Answer 3

HNNX 🐿 yes, predec is 16. Strange