single.php

<?php

/**
 * Get Related Posts
 * @see https://www.billerickson.net/related-posts-with-searchwp/
 * 
 */
function ea_get_related_posts( $related_to_show = false ) {
	$related_to_show = !empty( $related_to_show ) ? intval( $related_to_show ) : get_option( 'posts_per_page' );
	$related = array();

	// Use SearchWP
	if( class_exists( 'SearchWP_Related' ) && count( $related ) < $related_to_show ) {

		// Instantiate SearchWP Related
		$searchwp_related = new SearchWP_Related();

		// Use the keywords as defined in the SearchWP Related meta box
		$keywords = get_post_meta( get_the_ID(), $searchwp_related->meta_key, true );
		if( empty( $keywords ) )
			$keywords = get_the_title();

		$args = array(
			's'			=> $keywords,  // The stored keywords to use
			'engine'		=> 'default',  // the SearchWP engine to use
			'posts_per_page'	=> $related_to_show - count( $related ),
			'post__not_in'		=> array( get_the_ID() ),
			'fields'		=> 'ids',
		);

		// Retrieve Related content for the current post
		$swp_related = $searchwp_related->get( $args );
		if( !empty( $swp_related ) ) {
			$related = array_merge( $related, $swp_related );
		}
	}

	// Fallback, use primary category
	if( count( $related ) < $related_to_show ) {
		
		$loop = new WP_Query( array(
			'posts_per_page'	=> $related_to_show - count( $related ),
			'category_name'		=> ea_first_term( 'category', 'slug' ),
			'post__not_in'		=> array_merge( $related, array( get_the_ID() ) ),
			'fields' 		=> 'ids',
		));
		if( ! is_wp_error( $loop ) && !empty( $loop->posts ) )
			$related = array_merge( $related, $loop->posts );
	}

	return $related;
}