At the moment, I don't see a reliable way to test the persistence of a connection across supported platforms/drivers.

In pseudo-code, a test may look like following:

$conn = connect($persistent = false);
$id1 = $conn->id;

$conn = connect($persistent = true);
$id2 = $conn->id;

assertNotEquals($id1, $id2);

$conn = connect($persistent = true);
$id3 = $conn->id;

assertEquals($id2, $id3);

The results are:

1. The approach above works for MySQL (SELECT CONNECTION_ID()) and PostgreSQL (SELECT pg_backend_pid()).
2. SQL Server and Oracle return the same ID even for non-persistent connections (SELECT @@SPID and SELECT SYS_CONTEXT('USERENV', 'SID') FROM DUAL respectively).
3. I didn't find a way to obtain the connection ID for IBM DB2 and SQL Anywhere.

Can we at least test that persistent connection flag is active when persistent parameter is enabled?
For PDO, it can be done using PDO::getAttribute(). For mysqli, it can be done using mysqli::get_connection_stats().

Can we at least test that persistent connection flag is active when persistent parameter is enabled?
For PDO, it can be done using PDO::getAttribute().

Good idea. Done.

For mysqli, it can be done using mysqli::get_connection_stats().

If we use the active_persistent_connections key from there, the test may pass if there is a persistent connection in the server, not necessarily the current one. Let's keep the test skipped for non-PDO drivers.

Hi, @morozov, can you please backport this fix to upcoming 2.9.3 or 2.10.0?
It looks like small, but very useful feature.

No, it depends on #3080 which is a breaking change.