Can I use all PHP functions in WordPress?

Yes, You can use all the php functions as wordpress is in PHP. But its food to use the framework Functions and utilize them at right place instead of creating new functions and embedding them into it.

I am not sure but I think yes you can.

I agree with the answers above.

I agree with Mr. Rehan and Mr. Abul Razig answers.

https://codex.wordpress.org/Function_Reference

Agree with Rehan Farooq's Answer  

Very interesting question. Now I've got additional information. Thanks a lot and fully agreed with all answers.

Yes you can, but as what my colleagues answered above, it's always better to use WordPress framework functions. If you didn't find the functionality that you are looking for within the WordPress framework you can build your own function and integrate it with the core WordPress by using something in WordPress called Hooks. 

it could be so. ......................