PHP - strtotime, specify timezone

Laconic Answer (no need to change default timezone)

$dateStr = '2008-09-11';
$timezone = 'America/New_York';
$time = strtotime(
    $dateStr, 
    // convert timezone to offset seconds
    (new \DateTimeZone($timezone))->getOffset(new \DateTime) - (new \DateTimeZone(date_default_timezone_get()))->getOffset(new \DateTime) . ' seconds'
);

Loquacious Answer

Use strtotime's second option which changes the frame of reference of the function. By the way I prefer not to update the default time zone of the script:

http://php.net/manual/en/function.strtotime.php

int strtotime ( string $time [, int $now = time() ] )

The function expects to be given a string containing an English date format and will try to parse that format into a Unix timestamp (the number of seconds since January 1 1970 00:00:00 UTC), relative to the timestamp given in now, or the current time if now is not supplied.

And a Helper

/**
 * Returns the timestamp of the provided time string using a specific timezone as the reference
 * 
 * @param string $str
 * @param string $timezone
 * @return int number of the seconds
 */
function strtotimetz($str, $timezone)
{
    return strtotime(
        $str, strtotime(
            // convert timezone to offset seconds
            (new \DateTimeZone($timezone))->getOffset(new \DateTime) - (new \DateTimeZone(date_default_timezone_get()))->getOffset(new \DateTime) . ' seconds'
        )
    );
}

var_export(
    date(
        'Y-m-d', 
        strtotimetz('this monday', 'America/New_York')
    )
);

Maybe not the most performant approach, but works well when you know the default timezone and the offset. For example if the default timezone is UTC and the offset is -8 hours:

var_dump(
    date(
        'Y-m-d', 
        strtotime('this tuesday', strtotime(-8 . ' hours'))
    )
);