WP_REST_Posts_Controller::create_item PHP Method

    public function create_item($request)
    {
        if (!empty($request['id'])) {
            return new WP_Error('rest_post_exists', __('Cannot create existing post.'), array('status' => 400));
        }
        $prepared_post = $this->prepare_item_for_database($request);
        if (is_wp_error($prepared_post)) {
            return $prepared_post;
        }
        $prepared_post->post_type = $this->post_type;
        $post_id = wp_insert_post(wp_slash((array) $prepared_post), true);
        if (is_wp_error($post_id)) {
            if ('db_insert_error' === $post_id->get_error_code()) {
                $post_id->add_data(array('status' => 500));
            } else {
                $post_id->add_data(array('status' => 400));
            }
            return $post_id;
        }
        $post = get_post($post_id);
        /**
         * Fires after a single post is created or updated via the REST API.
         *
         * The dynamic portion of the hook name, `$this->post_type`, refers to the post type slug.
         *
         * @since 4.7.0
         *
         * @param WP_Post         $post     Inserted or updated post object.
         * @param WP_REST_Request $request  Request object.
         * @param bool            $creating True when creating a post, false when updating.
         */
        do_action("rest_insert_{$this->post_type}", $post, $request, true);
        $schema = $this->get_item_schema();
        if (!empty($schema['properties']['sticky'])) {
            if (!empty($request['sticky'])) {
                stick_post($post_id);
            } else {
                unstick_post($post_id);
            }
        }
        if (!empty($schema['properties']['featured_media']) && isset($request['featured_media'])) {
            $this->handle_featured_media($request['featured_media'], $post_id);
        }
        if (!empty($schema['properties']['format']) && !empty($request['format'])) {
            set_post_format($post, $request['format']);
        }
        if (!empty($schema['properties']['template']) && isset($request['template'])) {
            $this->handle_template($request['template'], $post_id);
        }
        $terms_update = $this->handle_terms($post_id, $request);
        if (is_wp_error($terms_update)) {
            return $terms_update;
        }
        if (!empty($schema['properties']['meta']) && isset($request['meta'])) {
            $meta_update = $this->meta->update_value($request['meta'], $post_id);
            if (is_wp_error($meta_update)) {
                return $meta_update;
            }
        }
        $post = get_post($post_id);
        $fields_update = $this->update_additional_fields_for_object($post, $request);
        if (is_wp_error($fields_update)) {
            return $fields_update;
        }
        $request->set_param('context', 'edit');
        $response = $this->prepare_item_for_response($post, $request);
        $response = rest_ensure_response($response);
        $response->set_status(201);
        $response->header('Location', rest_url(sprintf('%s/%s/%d', $this->namespace, $this->rest_base, $post_id)));
        return $response;
    }