form.html

<form action="{{form_uri}}" method="post">
  <label for="ort">Ort</label>
  <input type="text" name="ort" id="ort" value="{{ort}}">
  <input type="submit">
</form>

template.php

<?php
$formular = file_get_contents('form.html');

$array = array('form_uri' => 'do_something.php', 'ort' => 'automatisch eingesetzter Ort');

foreach ($array as $key => $value) {
  $formular = str_replace('{{'.$key.'}}', $value, $formular);
}

echo $formular;