add_option( string $option, mixed $value = '', string $deprecated = '', string|bool $autoload = 'yes' )

function add_option( $option, $value = '', $deprecated = '', $autoload = null ) {
    global $wpdb;

    if ( ! empty( $deprecated ) ) {
        _deprecated_argument( __FUNCTION__, '2.3.0' );
    }

    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 add_option( $deprecated_keys[ $option ], $value, $deprecated, $autoload );
    }

    wp_protect_special_option( $option );

    if ( is_object( $value ) ) {
        $value = clone $value;
    }

    $value = sanitize_option( $option, $value );

    /*
     * Make sure the option doesn't already exist.
     * We can check the 'notoptions' cache before we ask for a DB query.
     */
    $notoptions = wp_cache_get( 'notoptions', 'options' );

    if ( ! is_array( $notoptions ) || ! isset( $notoptions[ $option ] ) ) {
        /** This filter is documented in wp-includes/option.php */
        if ( apply_filters( "default_option_{$option}", false, $option, false ) !== get_option( $option ) ) {
            return false;
        }
    }

    $serialized_value = maybe_serialize( $value );

    $autoload = wp_determine_option_autoload_value( $option, $value, $serialized_value, $autoload );

    /**
     * Fires before an option is added.
     *
     * @since 2.9.0
     *
     * @param string $option Name of the option to add.
     * @param mixed  $value  Value of the option.
     */
    do_action( 'add_option', $option, $value );

    $result = $wpdb->query( $wpdb->prepare( "INSERT INTO `$wpdb->options` (`option_name`, `option_value`, `autoload`) VALUES (%s, %s, %s) ON DUPLICATE KEY UPDATE `option_name` = VALUES(`option_name`), `option_value` = VALUES(`option_value`), `autoload` = VALUES(`autoload`)", $option, $serialized_value, $autoload ) );
    if ( ! $result ) {
        return false;
    }

    if ( ! wp_installing() ) {
        if ( in_array( $autoload, wp_autoload_values_to_autoload(), true ) ) {
            $alloptions            = wp_load_alloptions( true );
            $alloptions[ $option ] = $serialized_value;
            wp_cache_set( 'alloptions', $alloptions, 'options' );
        } else {
            wp_cache_set( $option, $serialized_value, 'options' );
        }
    }

    // This option exists now.
    $notoptions = wp_cache_get( 'notoptions', 'options' ); // Yes, again... we need it to be fresh.

    if ( is_array( $notoptions ) && isset( $notoptions[ $option ] ) ) {
        unset( $notoptions[ $option ] );
        wp_cache_set( 'notoptions', $notoptions, 'options' );
    }

    /**
     * Fires after a specific option has been added.
     *
     * The dynamic portion of the hook name, `$option`, refers to the option name.
     *
     * @since 2.5.0 As "add_option_{$name}"
     * @since 3.0.0
     *
     * @param string $option Name of the option to add.
     * @param mixed  $value  Value of the option.
     */
    do_action( "add_option_{$option}", $option, $value );

    /**
     * Fires after an option has been added.
     *
     * @since 2.9.0
     *
     * @param string $option Name of the added option.
     * @param mixed  $value  Value of the option.
     */
    do_action( 'added_option', $option, $value );

    return true;
}