Description:
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I've ran onto problem described in bug #34988

I aknowledge that there is workaround by setting appropriate socket in DSN, however the root cause is that PHP defaults to socket='', which certainly won't work. 

If proper socket can't be found, it would be wiser to fall back to built-in default or fail configure script and require user to supply socket via something like --with-pdo-mysql-socket.

Requirement to use unix_socket in DSN ties code with machine-dependent configuration and AFAIK it's not possible to find that PDO has empty string for default socket without connection attempt - that makes it imperfect workaround.