Lazy loading class inside class

lazy_loading_example.php

<?php

class book
{
	// Singular author
	private $_author = null;
	public function author()
	{
		if(is_null($this->_author))
		{
			$this->_author = new Author($this->author_id);
		}

		return $this->_author;
	}

	// Multiple authors
	private $_authors = array();
	public function authors()
	{
		if(empty($this->_authors))
		{
			$this->_authors = getAuthorsFromDb();
			// getAuthorsFromDb is a bs function for loading up an array of models. replace it with something usable :)
		}

		return $this->_authors;
	}
}