You should know $smarty->security.
When this setting is on, the feature of "function modifier" will be disabled other than written in $smarty->security_settings['MODIFIER_FUNCS']
In Smarty's code:
* This enables template security. When enabled, many things are restricted
* in the templates that normally would go unchecked. This is useful when
* untrusted parties are editing templates and you want a reasonable level
* of security. (no direct execution of PHP in templates for example)