XTemplate is a cool template engine for PHP. The template engine allows you to store your HTML code separately from your PHP code (as opposed to compiling your template into PHP as per Smarty etc.). XTemplate has many useful features such as nested blocks and various kinds of variable interpolation, and yet the code is very short and very optimized.
XTemplate has been around for several years, it is considered stable and mature and has been used in many projects, from personal to larger commercial and open source ones. Last update of XTemplate is from 2008. Well, this is not because the project is not maintained. Just it works really great.
XTemplate supports also the older PHP4 (if anyone still uses it).