Placeholder characters in template that will not break code

[ColinK]

I plan to use a template PHP page which will include placeholders, some of which may be replaced using search and replace.

I want to choose a character to surround some code which may be replaced, but want to ensure that if some placeholders are not updated that code will still work.

These placeholders could be on any part of the page

Will any of the following characters work (replacing !! In example below)? !,%,+

Is there a better / safer character or pair of characters to use?

I know // or ## could break code if left on final page

I would like to use a character or pair of characters that are more visible to the human eye than !
Note - I cannot figure out how to display my code in this forum as 5 separate lines

<?php $roses = "!!roses!!"; // !!roses!! Will be replaced by red $violets = "!!violets!!"; // !!violets!! May or may not be replaced by blue echo "Roses are $roses"; ?>

Thanks ColinK