I heard that is possible to do that with left4dhooks vscripts functions, but I was never able to make it run.
One example from left4dhooks_test
PHP Code:
char buffer[128];
Format(buffer, sizeof(buffer), "GetCurrentFlowPercentForPlayer(%d)", client);
if( L4D2_GetVScriptOutput(buffer, buffer, sizeof(buffer)) )
PrintToServer("VScript: GetCurrentFlowPercentForPlayer(%d) = %s", client, buffer);