The title is quite self explanatory. This is an example of the function I wish to call within my Smarty template without the {php}{/php} tags:
form_input(array('name' => 'username', 'id' => 'username', 'value' => set_value('username')))
The smarty error I am getting is
Unexpected " => ", expected one of: "","" , ")"
In case this is what you're using in Smarty, keep in mind that you need to use bracket syntax for arrays (i.e. {form_input(['name' => 'username', 'id' => 'username', 'value' => {set_value('username')}])}, or something closer to that)