php strtotime() 日期转换为时间戳 UTC时区

print strtotime($date." UTC"); 

OR

echo date_default_timezone_get();
output: America/Chicago

echo gmdate('Y-m-d H:i:s', strtotime('2011-10-27T20:23:39'));
output: 2011-10-28 01:23:39

echo gmdate('Y-m-d H:i:s', strtotime('2011-10-27T20:23:39 America/Chicago'));
output: 2011-10-28 01:23:39

echo gmdate('Y-m-d H:i:s', strtotime('2011-10-27T20:23:39 UTC'));
output 2011-10-27 20:23:39