Basic Syntax: Attributes

Attributes
Most of the functions take attributes that specify or modify their behavior. Attributes to Smarty functions are much like HTML attributes. Static values don't have to be enclosed in quotes, but it is required for literal strings. Variables with or without modifiers may also be used, and should not be in quotes. You can even use PHP function results, plugin results and complex expressions.

Some attributes require boolean values (TRUE or FALSE). These can be specified as true and false. If an attribute has no value assigned it gets the default boolean value of true.

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https://www.smarty.net/docs/en/language.syntax.attributes.tpl