Description:
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DateTime's constructor is documented like this:

public DateTime::__construct ([ string $time = "now" [, DateTimeZone $timezone = NULL ]] )

However, not passing the constructor the second argument is not the same as passing NULL. Fixing this is just a matter of adding a "!", like this

if (SUCCESS == zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "|sO", &time_str, &time_str_len, &timezone_object, date_ce_timezone)) {

into

if (SUCCESS == zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "|sO!", &time_str, &time_str_len, &timezone_object, date_ce_timezone)) {

Test script:
---------------
<?php

date_default_timezone_set("Europe/Lisbon");
$a = new DateTime("2009-01-01", null);
echo $a->format(DateTime::COOKIE);

Expected result:
----------------
Thursday, 01-Jan-09 00:00:00 WET

Actual result:
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Fatal error: Uncaught exception 'Exception' with message 'DateTime::__construct() expects parameter 2 to be DateTimeZone, null given' in /tmp/cpCZPA42:4 Stack trace: #0 /tmp/cpCZPA42(4): DateTime->__construct('2009-01-01', NULL) #1 {main} thrown in /tmp/cpCZPA42 on line 4