$query="SELECT * FORM TABELLA WHERE 1 ";
if( strlen($marca)>0 ) $query.="AND MARCA LIKE '%$marca%'";
if( strlen($modello)>0 ) $query.="AND modello LIKE '%$modello%'";
...
$query="SELECT * FORM TABELLA WHERE 1 ";
if( strlen($marca)>0 ) $query.="AND MARCA LIKE '%$marca%'";
if( strlen($modello)>0 ) $query.="AND modello LIKE '%$modello%'";
...