Get PHP DateTime objects with fractional seconds and format them according to ISO 8601.

iso8601TimestampWithMilliseconds.php

/*
 * The `DateTime` constructor doesn't create objects with fractional seconds.
 * However, the static method `DateTime::createFromFormat()` does include the
 * fractional seconds in the object.  Finally, since ISO 8601 specifies only
 * millisecond precision, remove the last three decimal places from the timestamp.
 */

// DateTime object with microseconds
$now = DateTime::createFromFormat('U.u', number_format(microtime(true), 6, '.', '')); 

// Truncate to milliseconds
$nowFormatted = substr($now->format('Y-m-d\TH:i:s.u'), 0, -3) . 'Z';