How can I resolve smarty function in php?

How can I resolve smarty function in php?

I'm hoping someone can point me to the right direction in this.
Lets say I store Smarty template in database. It can look like this:
{currentusername UserID="5"}
The currentusername is a custom function to get the username. All displays
correctly when used in a template. But what if I wanted to get the
resolved currentusername function in my php code?
Basically I would get the template from the database, resolve it through
smarty and then used the currentusername further in the code.
Is that possible?