prepare sample PHP script (test.php):

<?php

error_reporting(-1);

echo '!!!';

header('Content-Type: text/plain');

enable Xdebug 2.1.0 in php.ini

run sample script from the command line:

php -f test.php
!!!

enable Xdebug 2.1.1 in php.ini

run sample script from the command line:

php -f test.php
!!!PHP Warning: Cannot modify header information - headers already sent by (output started at test.php:3) in test.php on line 5
PHP Stack trace:
PHP 1. {main}() test.php:0
PHP 2. header() test.php:5

Warning: Cannot modify header information - headers already sent by (output started at test.php:3) in test.php on line 5

Call Stack:
0.0016 323272 1. {main}() test.php:0
0.0020 323384 2. header() test.php:5