prova:

codice:
<td> 
<? (if $riga['eta'] == 0){ ?>
	<font size="2" face="Verdana, Arial, Helvetica, sans-serif">
	Dato non disponibile</font> 
<? } else{ ?>
	<font size="2" face="Verdana, Arial, Helvetica, sans-serif">
	<?= $riga['eta'] ?>
	</font>
<? } ?>
</td>