Defining html code inside PHP variables

Try this:

<?php ob_start(); ?>
<div>HTML goes here...</div>
<div>More HTML...</div>
<?php $my_var = ob_get_clean(); ?>

This way you will retain syntax highlighting for HTML.

<?php
$my_var = <<<EOD
<div>SOME HTML</div>
EOD;
?>

It's called heredoc syntax.