How to use custom code inside multiline strings?

Question

I am trying to run PHP code inside DELIMETER but it does not work.

I want to do a box to show all quantity available on my stock.

This is my code:

$product = <<<DELIMETER
<a class="btn btn-default">
<select id ="quantidade" name="quantidade"> <?PHP for ($i = 0; $i<=$row['produto_quantidade']; $i++) echo "<option value=".$i.">".$i." </option>";
?>  </select> </a>
DELIMETER;