Why does this return empty string ?

gistfile1.txt

		$message = file_get_contents(__DIR__ . '/confirmation.html');

		$replace = array(
			'[site_url]' => get_site_url() . '/wp-content/themes/artists-working/img/email/',
			'[email]' => $email,
			'[event_id]' => $event_id,
			'[address]' => $event_address
		);

		foreach ($replace as $placeholder => $info) {
			$message = str_replace($placeholder,$message, $info);
		}
		echo var_dump($message); // returns "" ?!?!