Templates and placeholders. Why does not it work?

Question

PHP

There is some page. It has placeholders like {message}.
When the user gets to the main page, a message about successful registration will appear at this place. But there is a problem - when there is no message, the placeholder is visible! I can't get it off the page. Make an array and loop through each time to remove not unnecessary - not an option, because. there are a lot of them and it is not convenient to edit this array every time.
I replace it with content like this:
$html = str_replace("{placeholder}", $content, $html);
The catch is that the code doesn't know the name of the placeholder, so the loop won't help.

Answer 1

This logic is not clear, why create unnecessary problems for yourself, but in this case I think preg_replace() will help .
That is, first you replace, and after preg_replace() you "pick up" the remaining placeholders
ps I hope I understood you correctly

Answer 2

looks like some bullshit.

<?php if ($message): ?>
  <div class="message"><?php echo $message; ?></div>
<?php endif; ?>

this is how you can do it.