Considering this question and answer
My question is, is PHP thread safe in 2012?
PHP can be thread-safe if you don't use non-thread-safe extensions. See this page for information on thread safety and checking that your PHP variant is thread safe for use with Apache. And have a look here if you need help choosing between thread-safe or non-thread-safe editions of PHP: it depends on the Apache worker and how it handles requests (in terms of threads, processes, etc.).