下面是源代码中wpdb类的构造函数。根据谷歌搜索，这一名称最近已从USE\\u EXT\\u MYSQL重命名。然而，我在源代码中找不到任何定义了常量的地方。

function __construct( $dbuser, $dbpassword, $dbname, $dbhost ) {
    register_shutdown_function( array( $this, \'__destruct\' ) );

    if ( WP_DEBUG && WP_DEBUG_DISPLAY )
        $this->show_errors();

    /* Use ext/mysqli if it exists and:
     *  - WP_USE_EXT_MYSQL is defined as false, or
     *  - We are a development version of WordPress, or
     *  - We are running PHP 5.5 or greater, or
     *  - ext/mysql is not loaded.
     */
    if ( function_exists( \'mysqli_connect\' ) ) {
        if ( defined( \'WP_USE_EXT_MYSQL\' ) ) {
            $this->use_mysqli = ! WP_USE_EXT_MYSQL;
        } elseif ( version_compare( phpversion(), \'5.5\', \'>=\' ) || ! function_exists( \'mysql_connect\' ) ) {
            $this->use_mysqli = true;
        } elseif ( false !== strpos( $GLOBALS[\'wp_version\'], \'-\' ) ) {
            $this->use_mysqli = true;
        }
    }

谢谢