<?php
class LazyPdo extends PDO
{
    private $tenantId;
    private $activeConnection;

    public function __construct($tenantId, Pdo $masterConnection)
    {
        $this->tenantId = $tenantId;
        $this->activeConnection = $masterConnection;
        $this->isMaster = true;
    }

    private function initTenantDb() {
        $connectionDetails = $this->activeConnection->query('SELECT blah');
        $this->activeConnection = new Pdo($connectionDetails);
        $this->isMaster = false
    }

    public function prepare($statement, array $driver_options = array())
    {
        if ($this->isMaster) $this->initTenantDb();
        $this->activeConnection->prepare($statement, $driver_options);
    }

    // all the other Pdo methods with the same if check and delegatation to activeConnection
}