计算2个日期之间的时间差（以秒为单位）[重复]

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I have to 2 dates values in PHP,

start_date_time = "2018-03-15T20:39:06Z"

end_date_time = "2018-03-17T12:42:08Z"

duration = ?  // in seconds

I actually want to get total time in seconds. Please help

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$start_date_time  = strtotime('2011-05-12 18:20:20');
$end_date_time = strtotime('2011-05-13 18:20:20');

$durationeInSeconds = $start_date_time - $end_date_time;