Creating ISO 8601-extended in PHP, using DateTime. This includes milliseconds (and if you want, microseconds). Based on the work mentioned here: http://stackoverflow.com/a/4414060/1061927

gistfile1.php

<?php

// Our input
$time = microtime(true);

// Determining the microsecond fraction
$microSeconds = sprintf("%06d", ($time - floor($time)) * 1000000);

// Creating our DT object
$tz = new DateTimeZone("Etc/UTC"); // NOT using a TZ yields the same result, and is actually quite a bit faster. This serves just as an example.
$dt = new DateTime(date('Y-m-d H:i:s.'. $microSeconds, $time), $tz);

// Compiling the date. Limiting to milliseconds, without rounding
$iso8601Date = sprintf(
    "%s%03d%s",
    $dt->format("Y-m-d\TH:i:s."),
    floor($dt->format("u")/1000),
    $dt->format("O")
);

// Formatting according to ISO 8601-extended
var_dump(
     $iso8601Date
);


/*  // Proving the flip-over:
    [..]
    string(28) "2013-06-26T13:43:21.999+0000"
    string(28) "2013-06-26T13:43:21.999+0000"
    string(28) "2013-06-26T13:43:22.000+0000"
    [..]
    string(28) "2013-06-26T13:43:22.000+0000"
    string(28) "2013-06-26T13:43:22.001+0000"
    string(28) "2013-06-26T13:43:22.001+0000"
*/

I've developer PHP-Jiffy many months ago to solve this problem in a more general way, maybe you can enjoy it:

• https://github.com/Litipk/php-jiffy