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smarty is dedicated to display, php to calculation/data generation.
so it's not recommended to add php in smarty html templates
there are smarty tags to add some php code in smarty templates, but not recommended at all.
smarty.net is your friend
http://smarty.net/manual/fr/language.function.php.php (French)
http://smarty.net/manual/en/language.function.php.php (English)
Do synergy or die.