PHP PDO class with lazy connect and fail-over to alternate database servers

FailoverPDO.php

<?php

class FailoverPDO extends PDO
{
    protected $dsns, $failoverDSNIdx, $username, $passwd, $options, $connectCallback;

    public $logConnectFailures = true;

    protected $attrs = [];
    protected $connectAttempted = false;
    protected $pdo = null;

    public function __construct($primaryDSNs, $failoverDSNs, $username = '', $passwd = '', $options = [], $connectCallback = null)
    {
        $primaryDSNs = (is_array($primaryDSNs) && (!empty($primaryDSNs))) ?
            array_unique($primaryDSNs) : [];
        $failoverDSNs = (is_array($failoverDSNs) && (!empty($failoverDSNs))) ?
            array_diff(array_unique($failoverDSNs), $primaryDSNs) : [];

        $this->failoverDSNIdx = count($primaryDSNs);
        $this->dsns = array_merge($primaryDSNs, $failoverDSNs);

        $this->username = $username;
        $this->passwd = $passwd;

        if (array_key_exists('logConnectFailures', $options)) {
            $this->logConnectFailures = (boolean)$options['logConnectFailures'];
            unset($options['logConnectFailures']);
        }

        $this->options = $options;

        $this->connectCallback = $connectCallback;
    } // __construct()

    public function beginTransaction()
    {
        if ($this->pdo === null) {
            $this->lazyConnect();
        }
        return $this->pdo->beginTransaction();
    } // beginTransaction()

    public function commit()
    {
        if ($this->pdo === null) {
            $this->lazyConnect();
        }
        return $this->pdo->commit();
    } // commit()

    public function errorCode()
    {
        if ($this->pdo === null) {
            $this->lazyConnect();
        }
        return $this->pdo->errorCode();
    } // errorCode()

    public function errorInfo()
    {
        if ($this->pdo === null) {
            $this->lazyConnect();
        }
        return $this->pdo->errorInfo();
    } // errorInfo()

    public function exec($statement)
    {
        if ($this->pdo === null) {
            $this->lazyConnect();
        }
        return $this->pdo->exec($statement);
    } // exec()

    public function getAttribute($attribute)
    {
        if ($this->pdo === null) {
            // If we're getting an attribute which we previously set, return that;
            // otherwise, initialize the backing PDO instance and get the attribute from it.
            if (array_key_exists($args[0], $this->attrs)) {
                return $this->attrs[$args[0]];
            }
            $this->lazyConnect();
        }
        return $this->pdo->getAttribute($attribute);
    } // getAttribute()

    public function inTransaction()
    {
        if ($this->pdo === null) {
            $this->lazyConnect();
        }
        return $this->pdo->inTransaction();
    } // inTransaction()

    public function lastInsertId($name = null)
    {
        if ($this->pdo === null) {
            $this->lazyConnect();
        }
        return $this->pdo->lastInsertId($name);
    } // lastInsertId()

    public function prepare($statement, $driver_options = [])
    {
        if ($this->pdo === null) {
            $this->lazyConnect();
        }
        return $this->pdo->prepare($statement, $driver_options);
    } // prepare()

    public function query(string $statement)
    {
        if ($this->pdo === null) {
            $this->lazyConnect();
        }
        return $this->pdo->query($statement);
    } // query()

    public function quote($string, $parameter_type = PDO::PARAM_STR)
    {
        if ($this->pdo === null) {
            $this->lazyConnect();
        }
        return $this->pdo->quote($string, $parameter_type);
    } // quote()

    public function rollBack()
    {
        if ($this->pdo === null) {
            $this->lazyConnect();
        }
        return $this->pdo->rollBack();
    } // rollBack()

    public function setAttribute($attribute, $value)
    {
        if ($this->pdo === null) {
            $this->attrs[$attribute] = $value;
            return true;
        }
        return $this->pdo->setAttribute($attribute, $value);
    } // setAttribute()

    protected function lazyConnect()
    {
        if ($this->connectAttempted) {
            throw new PDOException('Not connected');
        }

        $this->connectAttempted = true;

        for ($i = 0, $n = count($this->dsns); $i < $n; $i++) {
            try {
                $this->pdo = new PDO($this->dsns[$i], $this->username, $this->passwd, $this->options);
                foreach ($this->attrs as $attribute=>$value) {
                    $this->pdo->setAttribute($attribute, $value);
                }
                if (is_callable($this->connectCallback)) {
                    call_user_func_array($this->connectCallback, [$this, $this->dsns[$i], $i >= $this->failoverDSNIdx]);
                }
                return;
            } catch (PDOException $ex) {
                if (($i+1) >= $n) {
                    throw $ex;
                } else {
                    if ($this->logConnectFailures) {
                        error_log(sprintf('Connection to dsn %s failed; trying next dsn', $this->dsns[$i]));
                    }
                }
            }
        }
    } // lazyConnect()
}