Nothing is simulated, you are executing real threads.
You should not write the standard output from threads in SAPI mode, you will experience unexpected behaviour and errors, that cannot be controlled, there are too many environments and SAPI's to cover it well, so it is not covered at all, don't do it.
Even in CLI mode output of complex code will be garbled, to work around this, you can define a protected method in any object you pass to all contexts that takes care of writing standard output, if the method is protected and the object is a pthreads one, only one context will be able to write standard output at a time ... the same object could be used in the SAPI environment by swapping standard output for a logging database ...