Description:
------------
Because PHP_DBLIB doesn't implement pdo_dbh_check_liveness_func (its value is null, which is "equivalent to returning SUCCESS"), when instantiating a new PDO object using persistent connections with dblib, if the connection dies, PHP is not aware of it and any subsequent queries using the connection fail with "DBPROCESS is dead or not enabled".

pdo_dbh_check_liveness_func definition:
https://github.com/php/php-src/blob/caa710037e663fd78f67533b29611183090068b2/ext/pdo/php_pdo_driver.h#L264

pdo_dblib's implementation:
https://github.com/php/php-src/blob/caa710037e663fd78f67533b29611183090068b2/ext/pdo_dblib/dblib_driver.c#L417

In this specific example, a Microsoft SQL Server is used to simulate a connection timeout using the WAITFOR DELAY query (which causes the connection to be considered dead by dblib). Connection is made using FreeTDS 1.1.x.

Based on the source, seems to be an issue in PHP 8 as well.

Test script:
---------------
<?php
for( $i = 0; $i <= 1; $i++ )
{
    $pdo = new PDO( 'dblib:dbname=northwinds;host=contoso', 'username', 'password', [
        PDO::ATTR_PERSISTENT => false,
        PDO::ATTR_TIMEOUT    => 5
    ] );
    try
    {
        if( $i == 0 )
        {
            $pdo->query( "WAITFOR DELAY '00:00:10'" );
        }
        var_dump( $pdo->query( "SELECT 'output' AS Output" )->fetch( PDO::FETCH_ASSOC )['Output'] );
    }
    catch( Throwable $e )
    {
        var_dump( stripos( $pdo->errorInfo()[2], 'DBPROCESS is dead or not enabled' ) === 0 );
    }
    $pdo = null;
}
?>

Expected result:
----------------
bool(true)
string(6) "output"

Actual result:
--------------
bool(true)
bool(true)