How to convert RGB to LAB/XYZ?

Question

PHP

I can not find information on the translation of RGB -> LAB on the Internet.
The only thing I found was how to translate XYZ into LAB, but here's how RGB into XYZ, the devil knows.
Prompt the algorithm itself (and not a ready-made solution), please.

Answer 1

www.easyrgb.com/?X=MATH

Answer 2

www.brucelindbloom.com