Feel free to send a patch.

This would make more sense implemented in the DB lib and if DB connections are really that expensive maybe you should look at memcached or something simmilar.
Is there a reason you require pooling that the current pconnect stuff won`t make you happy?

Needs Closing.

The threading model that is in widest use does not lend itself to such things; Zend is meant to put up and tear down as quickly as possible, that includes mysql connections. 

PHP has tinkered with this sort of thing, persistent connections; it is always the wrong route to take.

Even where the threading model does lend itself to pooling connections, maybe in a fpm or apache2 worker environment, it is just out of the scope of PHP to implement the pooling, as suggested, and as in other languages (java springs to mind), the pooling should occur at the level of the driver, not at the level of the VM or extension (driver abstraction). Were such things available at the driver, we would then have a chance to make use of them, but it just impractical to implement right here.

Closing the request.