Vaiable locking does not occur nor really need to occur as there is no communication (through natural methods) between threads. All variables are re allocated for the child thread if modified.

If the threads wanted to comunicate (parent-child or child-child) the best way would be an IPC pipe or a dump file (like my serialized return result).

The reasoning behind php not supporting threading by default as it isnt neccearry for alot of things, its an extension like mysql. And just like mysql it can be used when not needed etc.