Description:
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From manual page: http://www.php.net/datetime.format
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DateTime::format() does not add leading zeros for the %i and %s format characters as documented.

https://secure.php.net/manual/en/datetime.format.php "Parameters/format" states "Format accepted by date()."  

https://secure.php.net/manual/en/datetime.format.php states 
- "I (capital i)	Whether or not the date is in daylight saving time" which is not minutes with leading zero
- 'S' is not in the list of format characters

The actual behavior of DateTime::format() is not properly documented.

Note that similar behavior/documentation dissonance exists for DateTimeInterval::format()


Test script:
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<?php

$time_start = new DateTime('now');
sleep(60*2+3.1415926535897961646);
$time_end = new DateTime('now');
$time_interval=$time_end->diff($time_start);
echo $time_interval->format("%H:%i:%s seconds") . PHP_EOL;
echo $time_interval->format("%H:%I:%S seconds") . PHP_EOL; // <- not documented for DateTime::format()