Currently there is no documentation available for IntlTimeZone::parse(), so it took some while to figure out a working example:

<?php
// Example for 32-bit PHP 7.0.30 with Intl version 1.1, ICU version 56.1
// To get the offset of a date including daylight saving time, getRawOffset() is
// not sufficient, getOffset() is needed. And a date is needed to take history
// in to account.

\Locale::setDefault('nl-NL');
$date = '25-03-2018 12:34:56.123456 CEST';  // 3rd of Februari
$timezone = \IntlTimeZone::createDefault();  // = CEST = GMT +02:00
print 'Time zone: ' . $timezone->getDisplayName() . PHP_EOL;
print 'DST: ' . ($timezone->useDaylightTime() ? 'DOES' : 'DOES NOT')
  . ' happen to this timezone' . PHP_EOL;
print 'Possible DST difference (usec): ' . $timezone->getDSTSavings() . PHP_EOL;

$formatter = new \IntlDateFormatter(
  \Locale::getDefault(),
  \IntlDateFormatter::MEDIUM,
  \IntlDateFormatter::MEDIUM,
  $timezone
);
$timestamp = $formatter->parse($date);
if (FALSE === $timestamp) {
  throw new \Exception('Can not parse date');
}
elseif (is_float($timestamp)) {
  throw new \Exception('Y2K38 bug @ 32 bit, please use 64 bit');
}
print 'Date parsed by IntlFormatter::parse(): ' . date('c', $timestamp)
  . PHP_EOL;

$cal = \IntlCalendar::createInstance();
$cal->setTimeZone($timezone);
$cal->setLenient(FALSE);
// set UNIX timestamp offset (1970-01-01 00:00:00.000000) and add the timestamp
$cal->set(1970, 0, 1, 0, 0, 0);
$cal->set(\IntlCalendar::FIELD_MILLISECOND, 0); // note: msec, not usec
$cal->add(\IntlCalendar::FIELD_SECOND, $timestamp); // note: no milliseconds
$cal->add(\IntlCalendar::FIELD_MILLISECOND, 124);  // hardcode for simplicity

header('Content-type: text/plain');
print 'Date constructed with IntlCalendar: ' . $formatter->format($cal)
  . PHP_EOL;
print 'Raw offset by getRawOffset(): ' . $timezone->getRawOffset() . PHP_EOL;

/**
 * Function IntlTimeZone::getOffset()
 * @link http://icu-project.org/apiref/icu4c/classicu_1_1TimeZone.html#afcbc1c48bf0b453b0123c4cb75d20e96
 * @param float $date
 *   moment in time for which to return offsets, in units of milliseconds from
 *   January 1, 1970 0:00 GMT, either GMT time or local wall time, depending on
 *   `local'.
 * @param bool $local
 *   if true, `date' is local wall time; otherwise it is in GMT time.
 * @param int &$rawOffset
 *   output parameter to receive the raw offset, that is, the offset not
 *   including DST adjustments
 * @param int &$dstOffset
 *   output parameter to receive the DST offset, that is, the offset to be added
 *   to `rawOffset' to obtain the total offset between local and GMT time. If
 *   DST is not in effect, this value is zero; otherwise it is a positive value,
 *   typically one hour.
 */

$rawOffset = NULL;
$dstOffset = NULL;
$timestamp *= 1000.0; // convert unix timestamp from secs to msecs
$timezone->getOffset($timestamp, $local = FALSE, $rawOffset, $dstOffset);
print 'Output of getOffset():' . PHP_EOL . json_encode([
  'rawOffset' => $rawOffset,
  'dstOffset' => $dstOffset
], JSON_PRETTY_PRINT) . PHP_EOL;
?>