PHP as template engine will not complain when you mix up your HTML syntax. It will let you forget to close tags, mis-nest them, etc.
PHP's output is not escaped by default, so unless you remember to rigorously add htmlspecialchars() everywhere, your site will have HTML injection (XSS) vulnerabilities.
Hello
These problems are much worse when you try to generate XHTML properly. It's not that you can't do that with plain PHP - of course you can - but that requires more effort and diligence.
That's why my recommendation is PHPTAL. OPT2 is OK too.
