is_dst.php

<?php
/**
 * @param string $date (yyyy-mm-dd format)
 * @param string $timezone
 *
 * @return bool|null
 */
function is_dst( $date, $timezone ) {
	try {
		$datetime = date_create();
		$datetime->setTimezone( timezone_open( $timezone ) );

		list( $year, $month, $date ) = explode( '-', $date );
		$datetime->setDate( $year, $month, $date );
		$datetime->setTime( 12, 0 );

		return (bool) $datetime->format( 'I' );
	} catch ( Exception $e ) {
		return null;
	}
}

var_dump( is_dst( '2018-02-01', 'America/Vancouver' ) ); // false
var_dump( is_dst( '2018-06-01', 'America/Vancouver' ) ); // true