This is a design. 
Connection parameters must be the same to reuse existing persistent connection. This design is useful when you have multiple transactions.

Yes, but my point is that imagine this situation:

40 different usernames for the database
32 maximum connections to the database

It doesn't take much to figure out what's going to happen.  As soon as the first 32 usernames to be used have created persistent connections, the further 8 usernames can never pconnect -> and remember, that doesn't take into account the fact that there are multiple PHP processes running.

So, what I am suggesting is that once all 32 connections are being used and PHP then attempts to use a 33rd username, instead of refusing to open the connection you should destroy the least recently used persistent connection.  This could be inefficient, but at least it won't lock PHP out of Postgres.

Chris

I understand your problem, but I think resuing existing connection that has  different connection parameter will be a cause of hard to find bug or even a security hole.

Older PostgreSQL accepts really few number of connections. 7.1.x supports upto 1024 connections by default (You need to edit postgresql.conf)   I suggest to upgrade your server....

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Yasuo Ohgaki

I don't propose re-using an existing connection with different parameters at all!  What I'm saying is that if PHP hits the max database pconnections, and it needs to open another, then it should destroy an old one, and then create the new one, rather than just dying.

I use PHP 7.1.3, with 64 simultaneous connections.  It's not feasible to up to 1024 connections as you need to allocate a heap of shared memory buffers, equivalent to double the max connections.

Anyway, the solution is not to tell someone to 'increase their max connections', the solution is to fix the problem...

Does other db module close persistent connection if number of persistent connections reache max persistent connections? Closing connection can be a problem, though.

Anyway, If you use all connection to pgsql with persistent connection, you cannot make any new connections obviously.  Therefore,  there is php.ini entry to limit max number of persistent connections.

If other db modules close persistent connections when max persistent connection is reached, I'll work on it.  (If it feasible)