[WordPress] If the given post is a single post, then add a class to this post.

add-class-to-single-post.php

<?php 
/**
 * If the given post is a single post, then add a class to this post.
 *
 * @param    array       $classes    The array of classes being rendered on a single post element.
 * @return   array       $classes    The array of classes being rendered on a single post element.
 * @package  example
 * @since    1.0.0
 */
function example_add_post_class_to_single_post( $classes ) {

	if ( is_single() ) {
		array_push( $classes, 'single-post' );
	} // end if

	return $classes;

}
add_filter( 'post_class', 'example_add_post_class_to_single_post' );