Converting a DATETIME variable into a UNIX timestamp

Here’s how:

/**
 * convert a datetime value (Y-m-d H:i:s) to UNIX timestamp value
 * @param datetime $datetime
 * @return timestamp
 */
function datetime_to_timestamp($datetime) {
    list($date, $time) = explode(" ", $datetime);
    list($year, $month, $day) = explode("-", $date);
    list($hour, $minute, $second) = explode(":", $time);
    return mktime($hour, $minute, $second, $month, $day, $year);
}