Smarty allows access to PHP objects through the templates. There are two ways to access them.
One way is to register objects to the template, then use access them via syntax similar to custom functions.
The other way is to assign() objects to the templates and access them much like any other assigned variable.
The first method has a much nicer template syntax. It is also more secure, as a registered object can be restricted to certain methods or properties. However, a registered object cannot be looped over or assigned in arrays of objects, etc. The method you choose will be determined by your needs, but use the first method whenever possible to keep template syntax to a minimum.
If $security is enabled, no private methods or functions can be accessed (begininning with '_'). If a method and property of the same name exist, the method will be used.
You can restrict the methods and properties that can be accessed by listing them in an array as the third registration parameter.
By default, parameters passed to objects through the templates are passed the same way custom functions get them. An associative array is passed as the first parameter, and the smarty object as the second. If you want the parameters passed one at a time for each argument like traditional object parameter passing, set the fourth registration parameter to FALSE.
The optional fifth parameter has only effect with format being TRUE and contains a list of methods that should be treated as blocks. That means these methods have a closing tag in the template ({foobar->meth2}...{/foobar->meth2}) and the parameters to the methods have the same synopsis as the parameters for block-function-plugins: They get the four parameters $params, $content, &$smarty and &$repeat and they also behave like block-function-plugins.
See also register_object() and assign().