Use of "<<<" in PHP

I see in some code I'm looking at the following syntax which I've never used before; it's clear that it's stuffing everything between <<<HTM and HTM into the variable, but does anyone have links to good documentation on this syntax? Google fails completely on this one.

<?php
			$haddresses[$address[0]] = <<<HTM
			<div class="staff-user-contact">
				<div class="address-location"><span class="label-bold">{$address[0]}</span></div>
				<div class="address-office">{$address[1]}</div>
				<div class="address-address">{$address[4]}</div>
				$phone
				$mobile
			</div>
HTM;