Ok i solved it!
i added an int output parameter called last_id, before the end tag i included this statement:
select last_insert_id() into last_id;
In PHP i execute:
$db -> execQuery('call aggiungi_categoria('.$parametri['id_categoria_principale'].',"'.$parametri['nome'].'","'.$parametri['sigla'].'",'.$parametri['stato'].',@lastId);');
and finally
$lid = $db ->execQuery('select @lastId;');