Recommend a good resource for learning in depth about parallel processing in php?

I've found several options through googling:

• php7 pthreads
• something like exec(), proc_open(), pcntl_fork() to start multiple scripts in the background
• making multiple ajax requests from browser to an api to kick off several processes at once
• gearman?

I'm just looking for a very in depth background on everything parallel processing in php. maybe a textbook, or online class?

P.S. here are a few examples of some of the types of questions I have right now:

• how many tasks can I run in parallel using the exec() and similar methods? hardware requirements?
• how does the performance compare when running in parallel using these methods vs sequential
• even basic questions like difference between multithread, multiprocess?
• maybe examples of situations when each option makes the most sense/limitations of each method.
• plenty of coding examples!