In the Web you can find a lot of solutions to emulate multithreading in php. Most often they are based on the forks, but there are variations about using curl, proc_open and etc.

I did not like the alternatives that I found, so I had to write my own solution.
Here is the following set of requirements:

• Use of the forks
• In-sync state with the interface in the absence of the necessary extensions
• Multiple use of child processes
• A full data exchange between processes. That is, running with the arguments and getting results at the end
• The event exchange between the child process-"thread" and the basic process at work
• Handling the thread pool with the multiple use, transferring arguments and getting results
• Error handling
• Timeouts for the work performance, waiting for work thread, initialization
• Maximum performance

The result is a CThread library.

Here is a link to the sources:
xially 7 february 2012, 13:10