<?php
define('PREFIX', '/holiday');
define('SUFFIX', '/work');
define('BLABLA', '/lorem');
define('ETC', '/ipsum');
$cname = 'constant'; // if you want to use a function in heredoc, you must save function name in variable
$body = <<<EOD
<img src="{$cname(PREFIX)}/images/hello.png" />
<img src="{$cname(SUFFIX)}/images/hello.png" />
<img src="{$cname(BLABLA)}/images/hello.png" />
<img src="{$cname(ETC)}/images/hello.png" />
EOD;