Description:
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Currently session_start blocks until all previous session_start's are closed and there is no "at least not known to me" way to limit this blocking as even max_execution_time is not counted during this time.

So I would like to see session_start to accept a timout value like 

session_start(5) 

that returns, in this case, after 5 seconds and gives the programmer the chance to react to the fact that there is one/multiple long running session using the same session ID already.