As deceze said, you can use the PER_COUNTRY flag.
$timezone_identifiers = DateTimeZone::listIdentifiers(DateTimeZone::PER_COUNTRY, 'US');
foreach($timezone_identifiers as $timezone_identifier) {
echo "$timezone_identifier\n";
}
Output:
America/Adak
America/Anchorage
America/Boise
America/Chicago
America/Denver
America/Detroit
America/Indiana/Indianapolis
America/Indiana/Knox
America/Indiana/Marengo
America/Indiana/Petersburg
America/Indiana/Tell_City
America/Indiana/Vevay
America/Indiana/Vincennes
America/Indiana/Winamac
America/Juneau
America/Kentucky/Louisville
America/Kentucky/Monticello
America/Los_Angeles
America/Menominee
America/Metlakatla
America/New_York
America/Nome
America/North_Dakota/Beulah
America/North_Dakota/Center
America/North_Dakota/New_Salem
America/Phoenix
America/Shiprock
America/Sitka
America/Yakutat
Pacific/Honolulu
