get_option( string $option, mixed $default = false )

function get_option( $option, $default_value = false ) {
    global $wpdb;

    if ( is_scalar( $option ) ) {
        $option = trim( $option );
    }

    if ( empty( $option ) ) {
        return false;
    }

    /*
     * Until a proper _deprecated_option() function can be introduced,
     * redirect requests to deprecated keys to the new, correct ones.
     */
    $deprecated_keys = array(
        'blacklist_keys'    => 'disallowed_keys',
        'comment_whitelist' => 'comment_previously_approved',
    );

    if ( isset( $deprecated_keys[ $option ] ) && ! wp_installing() ) {
        _deprecated_argument(
            __FUNCTION__,
            '5.5.0',
            sprintf(
                /* translators: 1: Deprecated option key, 2: New option key. */
                __( 'The "%1$s" option key has been renamed to "%2$s".' ),
                $option,
                $deprecated_keys[ $option ]
            )
        );
        return get_option( $deprecated_keys[ $option ], $default_value );
    }

    /**
     * Filters the value of an existing option before it is retrieved.
     *
     * The dynamic portion of the hook name, `$option`, refers to the option name.
     *
     * Returning a value other than false from the filter will short-circuit retrieval
     * and return that value instead.
     *
     * @since 1.5.0
     * @since 4.4.0 The `$option` parameter was added.
     * @since 4.9.0 The `$default_value` parameter was added.
     *
     * @param mixed  $pre_option    The value to return instead of the option value. This differs from
     *                              `$default_value`, which is used as the fallback value in the event
     *                              the option doesn't exist elsewhere in get_option().
     *                              Default false (to skip past the short-circuit).
     * @param string $option        Option name.
     * @param mixed  $default_value The fallback value to return if the option does not exist.
     *                              Default false.
     */
    $pre = apply_filters( "pre_option_{$option}", false, $option, $default_value );

    /**
     * Filters the value of all existing options before it is retrieved.
     *
     * Returning a truthy value from the filter will effectively short-circuit retrieval
     * and return the passed value instead.
     *
     * @since 6.1.0
     *
     * @param mixed  $pre_option    The value to return instead of the option value. This differs from
     *                              `$default_value`, which is used as the fallback value in the event
     *                              the option doesn't exist elsewhere in get_option().
     *                              Default false (to skip past the short-circuit).
     * @param string $option        Name of the option.
     * @param mixed  $default_value The fallback value to return if the option does not exist.
     *                              Default false.
     */
    $pre = apply_filters( 'pre_option', $pre, $option, $default_value );

    if ( false !== $pre ) {
        return $pre;
    }

    if ( defined( 'WP_SETUP_CONFIG' ) ) {
        return false;
    }

    // Distinguish between `false` as a default, and not passing one.
    $passed_default = func_num_args() > 1;

    if ( ! wp_installing() ) {
        $alloptions = wp_load_alloptions();

        if ( isset( $alloptions[ $option ] ) ) {
            $value = $alloptions[ $option ];
        } else {
            $value = wp_cache_get( $option, 'options' );

            if ( false === $value ) {
                // Prevent non-existent options from triggering multiple queries.
                $notoptions = wp_cache_get( 'notoptions', 'options' );

                // Prevent non-existent `notoptions` key from triggering multiple key lookups.
                if ( ! is_array( $notoptions ) ) {
                    $notoptions = array();
                    wp_cache_set( 'notoptions', $notoptions, 'options' );
                } elseif ( isset( $notoptions[ $option ] ) ) {
                    /**
                     * Filters the default value for an option.
                     *
                     * The dynamic portion of the hook name, `$option`, refers to the option name.
                     *
                     * @since 3.4.0
                     * @since 4.4.0 The `$option` parameter was added.
                     * @since 4.7.0 The `$passed_default` parameter was added to distinguish between a `false` value and the default parameter value.
                     *
                     * @param mixed  $default_value  The default value to return if the option does not exist
                     *                               in the database.
                     * @param string $option         Option name.
                     * @param bool   $passed_default Was `get_option()` passed a default value?
                     */
                    return apply_filters( "default_option_{$option}", $default_value, $option, $passed_default );
                }

                $row = $wpdb->get_row( $wpdb->prepare( "SELECT option_value FROM $wpdb->options WHERE option_name = %s LIMIT 1", $option ) );

                // Has to be get_row() instead of get_var() because of funkiness with 0, false, null values.
                if ( is_object( $row ) ) {
                    $value = $row->option_value;
                    wp_cache_add( $option, $value, 'options' );
                } else { // Option does not exist, so we must cache its non-existence.
                    $notoptions[ $option ] = true;
                    wp_cache_set( 'notoptions', $notoptions, 'options' );

                    /** This filter is documented in wp-includes/option.php */
                    return apply_filters( "default_option_{$option}", $default_value, $option, $passed_default );
                }
            }
        }
    } else {
        $suppress = $wpdb->suppress_errors();
        $row      = $wpdb->get_row( $wpdb->prepare( "SELECT option_value FROM $wpdb->options WHERE option_name = %s LIMIT 1", $option ) );
        $wpdb->suppress_errors( $suppress );

        if ( is_object( $row ) ) {
            $value = $row->option_value;
        } else {
            /** This filter is documented in wp-includes/option.php */
            return apply_filters( "default_option_{$option}", $default_value, $option, $passed_default );
        }
    }

    // If home is not set, use siteurl.
    if ( 'home' === $option && '' === $value ) {
        return get_option( 'siteurl' );
    }

    if ( in_array( $option, array( 'siteurl', 'home', 'category_base', 'tag_base' ), true ) ) {
        $value = untrailingslashit( $value );
    }

    /**
     * Filters the value of an existing option.
     *
     * The dynamic portion of the hook name, `$option`, refers to the option name.
     *
     * @since 1.5.0 As 'option_' . $setting
     * @since 3.0.0
     * @since 4.4.0 The `$option` parameter was added.
     *
     * @param mixed  $value  Value of the option. If stored serialized, it will be
     *                       unserialized prior to being returned.
     * @param string $option Option name.
     */
    return apply_filters( "option_{$option}", maybe_unserialize( $value ), $option );
}