the problem I'm having started to appear lately and it doesn't appear everywhere.
The front end (html/js) calls PHP pages. PHP in turn calls functions from a COM dll (vb6). The COM dll does use some lower level C++ .dlls in turns, though not sure this information is relevant.
Now, lately (this did not happen before), in one specific COM dll function, let's call it BringClients(), sometimes (apparently some user concurrency happening - race condition?) a global variable let's say ClientName which is used within BringClients() is not empty when the function is called, but instead it holds a value, which is set on function UpdateClients() when this function is called in a completely different instance of the COM dll, by a completely different user.
I don't know why this is happening. Aren't all COM dll variables empty when the .dll is initialized? I worked around it by manually emptying the variable on each initilization of the .dll, but should this even be happening? I haven't noticed a similar issue on other functions.
The OS is Windows server with IIS.
I don't know how PHP interacts with COM components, but my guess is you don't really initialize the DLL, you create objects. And your COM library is apartment-threaded, so you will get an isolated set of variables per thread.
But what happens if IIS uses the same thread later (from a pool) based upon whatever scheduling it uses to balance requests? Then any global/public module-level data variables will still be there.
So yes, if you need to ensure that those variables are not re-used you are going to have to clear them or re-architect to remove all globals.