How to crop image after rotation?

Question

PHP

Hello!
The school curriculum is completely forgotten :(
Here is a picture to understand the task (an image can be not only a square, but also a rectangle):
We have an image (yellow rectangle) that we rotate (green rectangle). We need to crop it along the red rectangle =)
Please tell me, how can I find out the indent (h) for cropping an image to "exclude" an empty space.
Thanks in advance :)