public function db_connect( $allow_bail = true ) {
    $this->is_mysql = true;

    /*
		 * Deprecated in 3.9+ when using MySQLi. No equivalent
		 * $new_link parameter exists for mysqli_* functions.
		 */
    $new_link     = defined( 'MYSQL_NEW_LINK' ) ? MYSQL_NEW_LINK : true;
    $client_flags = defined( 'MYSQL_CLIENT_FLAGS' ) ? MYSQL_CLIENT_FLAGS : 0;

    if ( $this->use_mysqli ) {
      $this->dbh = mysqli_init();

      $host    = $this->dbhost;
      $port    = null;
      $socket  = null;
      $is_ipv6 = false;

      $host_data = $this->wpdb::parse_db_host( $this->dbhost );
      if ( $host_data ) {
        list( $host, $port, $socket, $is_ipv6 ) = $host_data;
      }

      /*
			 * If using the `mysqlnd` library, the IPv6 address needs to be
			 * enclosed in square brackets, whereas it doesn't while using the
			 * `libmysqlclient` library.
			 * @see https://bugs.php.net/bug.php?id=67563
			 */
      if ( $is_ipv6 && extension_loaded( 'mysqlnd' ) ) {
        $host = "[$host]";
      }

      if ( WP_DEBUG ) {
        mysqli_real_connect( $this->dbh, $host, $this->dbuser, $this->dbpassword, null, $port, $socket, $client_flags );
      } else {
        // phpcs:ignore WordPress.PHP.NoSilencedErrors.Discouraged
        @mysqli_real_connect( $this->dbh, $host, $this->dbuser, $this->dbpassword, null, $port, $socket, $client_flags );
      }

      if ( $this->dbh->connect_errno ) {
        $this->dbh = null;

        /*
				 * It's possible ext/mysqli is misconfigured. Fall back to ext/mysql if:
				 *  - We haven't previously connected, and
				 *  - WP_USE_EXT_MYSQL isn't set to false, and
				 *  - ext/mysql is loaded.
				 */
        $attempt_fallback = true;

        if ( $this->has_connected ) {
          $attempt_fallback = false;
        } elseif ( defined( 'WP_USE_EXT_MYSQL' ) && ! WP_USE_EXT_MYSQL ) {
          $attempt_fallback = false;
        } elseif ( ! function_exists( 'mysql_connect' ) ) {
          $attempt_fallback = false;
        }

        if ( $attempt_fallback ) {
          $this->use_mysqli = false;
          return $this->wpdb::db_connect( $allow_bail );
        }
      }
    } else {
      if ( WP_DEBUG ) {
        $this->dbh = mysql_connect( $this->dbhost, $this->dbuser, $this->dbpassword, $new_link, $client_flags );
      } else {
        // phpcs:ignore WordPress.PHP.NoSilencedErrors.Discouraged
        $this->dbh = @mysql_connect( $this->dbhost, $this->dbuser, $this->dbpassword, $new_link, $client_flags );
      }
    }

    if ( ! $this->dbh && $allow_bail ) {
      wp_load_translations_early();

      // Load custom DB error template, if present.
      if ( file_exists( WP_CONTENT_DIR . '/db-error.php' ) ) {
        require_once WP_CONTENT_DIR . '/db-error.php';
        die();
      }

      $message = '<h1>' . __( 'Error establishing a database connection' ) . "</h1>\n";

      $message .= '<p>' . sprintf(
        /* translators: 1: wp-config.php, 2: Database host. */
        __( 'This either means that the username and password information in your %1$s file is incorrect or we can&#8217;t contact the database server at %2$s. This could mean your host&#8217;s database server is down.' ),
        '<code>wp-config.php</code>',
        '<code>' . htmlspecialchars( $this->dbhost, ENT_QUOTES ) . '</code>'
      ) . "</p>\n";

      $message .= "<ul>\n";
      $message .= '<li>' . __( 'Are you sure you have the correct username and password?' ) . "</li>\n";
      $message .= '<li>' . __( 'Are you sure you have typed the correct hostname?' ) . "</li>\n";
      $message .= '<li>' . __( 'Are you sure the database server is running?' ) . "</li>\n";
      $message .= "</ul>\n";

      $message .= '<p>' . sprintf(
        /* translators: %s: Support forums URL. */
        __( 'If you&#8217;re unsure what these terms mean you should probably contact your host. If you still need help you can always visit the <a href="%s">WordPress Support Forums</a>.' ),