Why should I use templating system in PHP?
The reasoning behind my question is: PHP itself is feature rich templating system, why should I install another template engine?
The only two pros I found so far are:
... and both are quite negligible when compared to cons.
Small example:
PHP
<h1><?=$title?></h1>
<ul>
<? foreach ($items as $item) {?>
<li><?=$item?></li>
<? } ?>
</ul>
Smarty
<h1>{$title}</h1>
<ul>
{foreach item=item from=$items}
<li>{$item}</li>
{/foreach}
</ul>
I really don't see any difference at all.
Yes, as you said, if you don't force yourself to use a templating engine inside PHP ( the templating engine ) it becomes easy to slip and stop separating concerns.
However, the same people who have problems separating concerns end up generating HTML and feeding it to smarty, or executing PHP code in Smarty, so Smarty's hardly solving your concern separation problem.
See also: