If you haven't read this, it is an outstanding explanation of how the two PHP bugs were connected together and turned into an exploit. It is written by one of the people behind the discovery as an enjoyable documentary followed by technical detail. The documentary section was hand illustrated by their wife in a wonderful metaphorical comic book format.
https://calif.io/php2shell/
Califphp2shell by Calif ResearchA tale of two PHP remote vulnerabilities
commit 7554c06