It works on the video, as they told, I created a file called posttypes.php into the theme folder containing this code:
<?php echo "Hello!"; ?>
I have made a file called functions.php put into the child theme cooking:
<?php include_once(ABSPATH . 'wp_content/themes/posttypes.php'); ?>
As a consequence, I should have visually outputten a Hello! This is not the case, I have the following error output:
Warning: include_once(C:\wamp\www\portfolio/wp_content/themes/posttypes.php) [<a href='function.include-once'>function.include-once</a>]: failed to open stream: No such file or directory in C:\wamp\www\portfolio\wp-content\themes\cooking\functions.php on line 2' and
Warning: include_once() [<a href='function.include'>function.include</a>]: Failed opening 'C:\wamp\www\portfolio/wp_content/themes/posttypes.php' for inclusion (include_path='.;C:\php\pear') in C:\wamp\www\portfolio\wp-content\themes\cooking\functions.php on line 2
What did I fail?