why the hello_world block doesn't work?
PermalinkEnvironment : wamp 2.5
concrete5.7.4.2

The problem is caused by a short open tag in form.php on line 20 column 98.
Line 20 with the short open tag:
<input type="checkbox" name="booleanfield" value="1" <?php if ($booleanfield) { ?>checked<? } ?>>
Change line 20 to this:
<input type="checkbox" name="booleanfield" value="1" <?php if ($booleanfield) { ?>checked<?php } ?>>
- "<?" is being changed to "<?php"
Is this concrete5's shortage? Complicated php tag makes it is easy to be wrong... Why not considering some template language like smarty or some what...
I am pretty confident that the short open tag was a typo.
"PHP also allows for short open tag <? (which is discouraged since it is only available if enabled using the short_open_tag php.ini configuration file directive, or if PHP was configured with the --enable-short-tags option)."
http://php.net/manual/en/language.basic-syntax.phptags.php...
There was a recent issue on GitHub to include twig templating. If you have an opinion on the topic, you might want to add it to the discussion.
https://github.com/concrete5/concrete5/issues/2717...