codice:
$str = '

<option value="ag">Agrigento</option>
<option value="al">Alessandria</option>
<option value="an">Ancona</option>
<option value="ao">Aosta</option>
<option value="ar">Arezzo</option>
<option value="ap">Ascoli Piceno</option>
<option value="at">Asti</option>
<option value="av">Avellino</option>
<option value="ba">Bari</option>
<option value="bl">Belluno</option>'; 

preg_match_all( '#<option value="(.*)">(.*)</option>#U', $str, $ris );

for( $i = 0, $j = count( $ris[1] ); $i < $j ; $i++ )
{
	$valori[$ris[1][$i]] = $ris[2][$i];
}
print_r( $valori );