In the process of some development he's been doing on various shared modules for PHP, Derick Rethans stumbled across an issue with using Valgrind to test his code:

While testing whether I correctly free all memory with Valgrind, I ran into the issue where I couldn't see the stack frames of where the memory leaks occurred in the extensions, and once I even ran into a Valgrind bug. The reason why Valgrind could not show the function names belonging to the stack frames is because PHP had already unloaded the shared extensions from memory.

A work-around he found was compiling the modules, but he wanted something "more correct" and less of a hassle. As a result he added a check for the ZEND_DONT_UNLOAD_MODULES environment flag to the PHP core to handles this case specifically. He includes a snippet of example code showing the Valgrind results with and without the flag.