This function is special to all dynamic loadable modules. Take a look at the creation via the ZEND_GET_MODULE macro first:
#if COMPILE_DL_FIRSTMOD ZEND_GET_MODULE(firstmod) #endif
The function implementation is surrounded by a conditional compilation statement. This is needed since the function get_module() is only required if your module is built as a dynamic extension. By specifying a definition of COMPILE_DL_FIRSTMOD in the compiler command (see above for a discussion of the compilation instructions required to build a dynamic extension), you can instruct your module whether you intend to build it as a dynamic extension or as a built-in module. If you want a built-in module, the implementation of get_module() is simply left out.
get_module() is called by Zend at load time of the module. You can think of it as being invoked by the dl() call in your script. Its purpose is to pass the module information block back to Zend in order to inform the engine about the module contents.
If you don't implement a get_module() function in your dynamic loadable module, Zend will compliment you with an error message when trying to access it.