In multi-threaded environments (like most web platforms) I often include some sort of thread ID to the logs of my apps. This enables me to tell exactly what log entry came from which request/thread, when there are multiple requests at once which are simultaneously writing to the same log.
In .NET/C#, this can be done by the formatters of log4net, which by default include the current thread's
int zend_thread_id ( void )
This function returns a unique identifier for the current thread.
This function is only available if PHP has been built with ZTS (Zend Thread Safety) support and debug mode (--enable-debug).