Display Related Posts by Category

global $post;

if ( ! empty( $post ) ) {

	$categories = get_the_category( $post->ID );

	if ( $categories ) {

		$category_ids = array();

		foreach ( $categories as $category ) {

			$category_ids[] = $category->term_id;

		}

		$query_args = array(

			'category__in'   => $category_ids,

			'post__not_in'   => array( $post->ID ),

			'posts_per_page' => 5

		);

		$related_posts = new WP_Query( $query_args );

		if ( $related_posts->have_posts() ) {

			echo '<div class="related-posts">';

			echo '<h3>Related Posts</h3><ul>';

			while ( $related_posts->have_posts() ) : $related_posts->the_post();

				echo '<li><a href="' . get_permalink() . '">' . get_the_title() . '</a></li>';

			endwhile;

			echo '</ul>';

			echo '</div>';

			wp_reset_postdata();

		}

	}

}