It uses individual
something.tpl.php files to theme Drupal's
theme_something() functions. Drupal's themeable functions are documented on the Drupal API site. Every file contains an HTML skeleton with some simple PHP statements for the dynamic data. Thus, PHPTemplate is an excellent choice for theming if you know a bit of PHP: with some basic PHP snippets, you can create advanced themes easily.
If you don't know PHP, then PHPTemplate can still be a good choice because only small bits of code are involved. They can just be copy/pasted into your template.
An extended Forum discussion provides some of the reasoning behind the creation of PHPTemplate.
Tips: Print out available variables
Print the variables array
Print the variables array with the HTML markup
print htmlspecialchars(print_r(get_defined_vars(), TRUE), ENT_QUOTES);