WordPress search functionality to include authors' posts in search result if the search keyword matches authors' name.

maybe_include_author_posts.php

<?php

if ( ! function_exists( 'cre_custom_search_query' ) ) {
	/**
	 * Creates custom search query
	 * @param WP_Query object
	 * @return posts object
	 */
	add_action( 'pre_get_posts', 'cre_custom_search_query', 100 );
	function cre_custom_search_query( $query ) {
		if ( is_search() ) {
			// Order by post id desc
			$query->set('orderby', 'post_date');
			$query->set('order', 'DESC');
			$authors = cre_authors_by_keyword( $query->get('s') );
			// include posts by author.
			if ( ! empty( $authors ) ) {
				$query->set('s', '');
				$query->set( 'author__in', $authors );
			}
		}
	}
}
if ( ! function_exists( 'cre_authors_by_keyword' ) ) {
	/**
	 * Returns authors/users user ids by searching against their usernames,
	 * matching provided keyword.
	 *
	 * @param string $keyword
	 * @return mixed|array
	 */
	function cre_authors_by_keyword( $keyword ) {
		if ( empty( $keyword ) ) {
			return null;
		}
		$keyword_ = explode( ' ', $keyword );
		$first_name = $keyword_[0] ? $keyword_[0] : false;
		$last_name  = $keyword_[1] ? $keyword_[1] : false;
		$authors = new WP_User_Query(
			[
				'has_published_posts' => true,
				'search_columns' => ['display_name', 'user_login', 'user_nicename', 'user_email'],
				'fields'         => ['ID'],
				'who'            => 'authors',
				'order'          => 'ASC',
				'orderby'        => 'display_name',
				'search'         => '*'.esc_attr( strtolower( str_replace( ' ', '', $keyword ) ) ).'*',
				'number'         => 10,
				'meta_query'     => [
					'relation'    => 'AND',
					[
						'relation'     => 'OR',
						[
							'key'     => 'first_name',
							'value'   => $first_name,
							'compare' => 'LIKE'
						],
						[
							'key'     => 'last_name',
							'value'   => $last_name,
							'compare' => 'LIKE'
						],
						[
							'key'     => 'description',
							'value'   => $keyword ,
							'compare' => 'LIKE'
						]
					],
					// https://codex.wordpress.org/Roles_and_Capabilities#User_Level_to_Role_Conversion
					[
						'relation'  => 'AND',
						[
							'key'     => 'wp_user_level',
							'value'   => [ 2, 3, 4, 5, 6, 7 ],
							'compare' => 'IN',
						]
					]
				]
			]
		);
		// var_dump( $authors ); die;
		if ( $authors->get_total() > 0 ) {
			return wp_list_pluck( $authors->get_results(), 'ID' );
		}
		return null;
	}
		/**
		 * Adds functionality to include `display_name` column to be included in search clolumn for WP_User_Query
		 *
		 * @return array
		 */
		add_filter( 'user_search_columns', function( $search_columns ) {
			$search_columns[] = 'display_name';
			return $search_columns;
		} );
}