http://php.net/manual/en/internals2.memory.tsrm.php

When PHP is built with Thread Safety enabled, the engine requires a way to isolate contexts from one another, such that the threads of a process may service individual requests without interference. TSRM is omnipotent within PHP, extension authors have to do very little in order to make sure their extensions can function in both a Thread Safe and Non Thread Safe architecture.

Thread-Safe Resource Manager