You may have seen the documentation, but you have missed {php}
:
The {php} tags allow PHP code to be embedded directly into the template. They will not be escaped, regardless of the $php_handling setting. This is for advanced users only, not normally needed and not recommended.
Emphasis mine, source: http://www.smarty.net/docsv2/en/language.function.php.tpl
Note that putting PHP in template code is the easiest way to shoot yourself in the foot - the main purpose of Smarty is to separate PHP code and HTML templates. In other words, the mere fact of using this tag is a serious red flag; in most cases, it is possible to fix the underlying issue, and avoid PHP inside the template altogether.