update_metadata( string $meta_type, int $object_id, string $meta_key, mixed $meta_value, mixed $prev_value = '' )

function update_metadata( $meta_type, $object_id, $meta_key, $meta_value, $prev_value = '' ) {
    global $wpdb;

    if ( ! $meta_type || ! $meta_key || ! is_numeric( $object_id ) ) {
        return false;
    }

    $object_id = absint( $object_id );
    if ( ! $object_id ) {
        return false;
    }

    $table = _get_meta_table( $meta_type );
    if ( ! $table ) {
        return false;
    }

    $meta_subtype = get_object_subtype( $meta_type, $object_id );

    $column    = sanitize_key( $meta_type . '_id' );
    $id_column = ( 'user' === $meta_type ) ? 'umeta_id' : 'meta_id';

    // expected_slashed ($meta_key)
    $raw_meta_key = $meta_key;
    $meta_key     = wp_unslash( $meta_key );
    $passed_value = $meta_value;
    $meta_value   = wp_unslash( $meta_value );
    $meta_value   = sanitize_meta( $meta_key, $meta_value, $meta_type, $meta_subtype );

    /**
     * Short-circuits updating metadata of a specific type.
     *
     * The dynamic portion of the hook name, `$meta_type`, refers to the meta object type
     * (post, comment, term, user, or any other type with an associated meta table).
     * Returning a non-null value will effectively short-circuit the function.
     *
     * Possible hook names include:
     *
     *  - `update_post_metadata`
     *  - `update_comment_metadata`
     *  - `update_term_metadata`
     *  - `update_user_metadata`
     *
     * @since 3.1.0
     *
     * @param null|bool $check      Whether to allow updating metadata for the given type.
     * @param int       $object_id  ID of the object metadata is for.
     * @param string    $meta_key   Metadata key.
     * @param mixed     $meta_value Metadata value. Must be serializable if non-scalar.
     * @param mixed     $prev_value Optional. Previous value to check before updating.
     *                              If specified, only update existing metadata entries with
     *                              this value. Otherwise, update all entries.
     */
    $check = apply_filters( "update_{$meta_type}_metadata", null, $object_id, $meta_key, $meta_value, $prev_value );
    if ( null !== $check ) {
        return (bool) $check;
    }

    // Compare existing value to new value if no prev value given and the key exists only once.
    if ( empty( $prev_value ) ) {
        $old_value = get_metadata_raw( $meta_type, $object_id, $meta_key );
        if ( is_countable( $old_value ) && count( $old_value ) === 1 ) {
            if ( $old_value[0] === $meta_value ) {
                return false;
            }
        }
    }

    $meta_ids = $wpdb->get_col( $wpdb->prepare( "SELECT $id_column FROM $table WHERE meta_key = %s AND $column = %d", $meta_key, $object_id ) );
    if ( empty( $meta_ids ) ) {
        return add_metadata( $meta_type, $object_id, $raw_meta_key, $passed_value );
    }

    $_meta_value = $meta_value;
    $meta_value  = maybe_serialize( $meta_value );

    $data  = compact( 'meta_value' );
    $where = array(
        $column    => $object_id,
        'meta_key' => $meta_key,
    );

    if ( ! empty( $prev_value ) ) {
        $prev_value          = maybe_serialize( $prev_value );
        $where['meta_value'] = $prev_value;
    }

    foreach ( $meta_ids as $meta_id ) {
        /**
         * Fires immediately before updating metadata of a specific type.
         *
         * The dynamic portion of the hook name, `$meta_type`, refers to the meta object type
         * (post, comment, term, user, or any other type with an associated meta table).
         *
         * Possible hook names include:
         *
         *  - `update_post_meta`
         *  - `update_comment_meta`
         *  - `update_term_meta`
         *  - `update_user_meta`
         *
         * @since 2.9.0
         *
         * @param int    $meta_id     ID of the metadata entry to update.
         * @param int    $object_id   ID of the object metadata is for.
         * @param string $meta_key    Metadata key.
         * @param mixed  $_meta_value Metadata value.
         */
        do_action( "update_{$meta_type}_meta", $meta_id, $object_id, $meta_key, $_meta_value );

        if ( 'post' === $meta_type ) {
            /**
             * Fires immediately before updating a post's metadata.
             *
             * @since 2.9.0
             *
             * @param int    $meta_id    ID of metadata entry to update.
             * @param int    $object_id  Post ID.
             * @param string $meta_key   Metadata key.
             * @param mixed  $meta_value Metadata value. This will be a PHP-serialized string representation of the value
             *                           if the value is an array, an object, or itself a PHP-serialized string.
             */
            do_action( 'update_postmeta', $meta_id, $object_id, $meta_key, $meta_value );
        }
    }

    $result = $wpdb->update( $table, $data, $where );
    if ( ! $result ) {
        return false;
    }

    wp_cache_delete( $object_id, $meta_type . '_meta' );

    foreach ( $meta_ids as $meta_id ) {
        /**
         * Fires immediately after updating metadata of a specific type.
         *
         * The dynamic portion of the hook name, `$meta_type`, refers to the meta object type
         * (post, comment, term, user, or any other type with an associated meta table).
         *
         * Possible hook names include:
         *
         *  - `updated_post_meta`
         *  - `updated_comment_meta`
         *  - `updated_term_meta`
         *  - `updated_user_meta`
         *
         * @since 2.9.0
         *
         * @param int    $meta_id     ID of updated metadata entry.
         * @param int    $object_id   ID of the object metadata is for.
         * @param string $meta_key    Metadata key.
         * @param mixed  $_meta_value Metadata value.
         */
        do_action( "updated_{$meta_type}_meta", $meta_id, $object_id, $meta_key, $_meta_value );

        if ( 'post' === $meta_type ) {
            /**
             * Fires immediately after updating a post's metadata.
             *
             * @since 2.9.0
             *
             * @param int    $meta_id    ID of updated metadata entry.
             * @param int    $object_id  Post ID.
             * @param string $meta_key   Metadata key.
             * @param mixed  $meta_value Metadata value. This will be a PHP-serialized string representation of the value
             *                           if the value is an array, an object, or itself a PHP-serialized string.
             */
            do_action( 'updated_postmeta', $meta_id, $object_id, $meta_key, $meta_value );
        }
    }

    return true;
}