add_metadata( string $meta_type, int $object_id, string $meta_key, mixed $meta_value, bool $unique = false )

function add_metadata( $meta_type, $object_id, $meta_key, $meta_value, $unique = false ) {
    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' );

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

    /**
     * Short-circuits adding 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:
     *
     *  - `add_post_metadata`
     *  - `add_comment_metadata`
     *  - `add_term_metadata`
     *  - `add_user_metadata`
     *
     * @since 3.1.0
     *
     * @param null|bool $check      Whether to allow adding 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 bool      $unique     Whether the specified meta key should be unique for the object.
     */
    $check = apply_filters( "add_{$meta_type}_metadata", null, $object_id, $meta_key, $meta_value, $unique );
    if ( null !== $check ) {
        return $check;
    }

    if ( $unique && $wpdb->get_var(
        $wpdb->prepare(
            "SELECT COUNT(*) FROM $table WHERE meta_key = %s AND $column = %d",
            $meta_key,
            $object_id
        )
    ) ) {
        return false;
    }

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

    /**
     * Fires immediately before meta of a specific type is added.
     *
     * 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:
     *
     *  - `add_post_meta`
     *  - `add_comment_meta`
     *  - `add_term_meta`
     *  - `add_user_meta`
     *
     * @since 3.1.0
     *
     * @param int    $object_id   ID of the object metadata is for.
     * @param string $meta_key    Metadata key.
     * @param mixed  $_meta_value Metadata value.
     */
    do_action( "add_{$meta_type}_meta", $object_id, $meta_key, $_meta_value );

    $result = $wpdb->insert(
        $table,
        array(
            $column      => $object_id,
            'meta_key'   => $meta_key,
            'meta_value' => $meta_value,
        )
    );

    if ( ! $result ) {
        return false;
    }

    $mid = (int) $wpdb->insert_id;

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

    /**
     * Fires immediately after meta of a specific type is added.
     *
     * 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:
     *
     *  - `added_post_meta`
     *  - `added_comment_meta`
     *  - `added_term_meta`
     *  - `added_user_meta`
     *
     * @since 2.9.0
     *
     * @param int    $mid         The meta ID after successful update.
     * @param int    $object_id   ID of the object metadata is for.
     * @param string $meta_key    Metadata key.
     * @param mixed  $_meta_value Metadata value.
     */
    do_action( "added_{$meta_type}_meta", $mid, $object_id, $meta_key, $_meta_value );

    return $mid;
}