Create CoAuthor for WordPress Coauthors Plus CAP Plugin

create-coauthor.php

<?php 
 /**
 * Creates a CoAuthor and returns the user_nicename.
 * Returns false if the author couldn't be created.
 *
 * @param $author_name string Author Name.
 *
 * @return false|string
 */
function create_and_get_author( $author_name ) {
	global $coauthors_plus;

	// Build the author name and slug.
	$author_slug = sanitize_title( $author_name );
	$author_tax_slug = 'cap-' . $author_slug;

	// Create the post name to be used for the guest author
	$post_name = $coauthors_plus->guest_authors->get_post_meta_key( $author_slug );

	// Create the user as a new guest
	$new_post = [
		'post_title' => $author_name,
		'post_name' => $post_name,
		'post_type' => $coauthors_plus->guest_authors->post_type,
		'post_status' => 'publish',
	];
	$user_post_id = wp_insert_post( $new_post, true );

	if ( is_wp_error( $user_post_id ) ) {
		return false;
	} else {
		// Add or update the necessary meta values
		update_post_meta( $user_post_id, $coauthors_plus->guest_authors->get_post_meta_key( 'display_name' ), $author_name );
		update_post_meta( $user_post_id, $coauthors_plus->guest_authors->get_post_meta_key( 'user_login' ), $author_slug );

		// Ensure there's an 'author' term for this user/guest author
		if ( ! wpcom_vip_term_exists( $author_slug, $coauthors_plus->coauthor_taxonomy ) ) {
			$author_term = wp_insert_term(
				$author_slug,
				$coauthors_plus->coauthor_taxonomy,
				[
					'slug' => $author_tax_slug,
				]
			);
			if ( ! is_wp_error( $author_term ) ) {
				$author = $coauthors_plus->get_coauthor_by( 'id', $user_post_id );
				return $author->user_nicename;
			} else {
				return false;
			}
		}
	}
	return false;
}