Apache 2.4: PHP files are not being sent to the browser - php

Question

I'm setting up a new site locally on a Windows machine for testing. In the document root, if I have an index.html it is served to the browser without problem. If I rename it index.php, the browser receives nothing. No error is raised server-side. I'm trying to understand why.
Vhosts
<VirtualHost *:80>
DocumentRoot "C:\websites\learn"
ServerName learn.loc
LogLevel alert rewrite:trace2
#PHP SETTINGS
php_value auto_prepend_file "C:\websites\learn\noop.php"
php_value open_basedir "C:\websites\learn"
<Directory "C:\websites\learn">
Options Indexes FollowSymLinks
AllowOverride All
Require all granted
</Directory>
</VirtualHost>
Here is the .htaccess file that resides in the document root:
RewriteEngine on
#point to javascript learning project
RewriteRule ^js /javascript
RewriteRule ^js/(.*) /javascript/$1
Here is the mod_rewrite log generated when I load learn.loc/javascript (this folder has an index.php file)
[initial] [perdir C:/websites/learn/] pass through C:/websites/learn/javascript/
[subreq] [perdir C:/websites/learn/] pass through C:/websites/learn/javascript/index.html
[subreq] [perdir C:/websites/learn/] pass through C:/websites/learn/javascript/index.htm
[subreq] [perdir C:/websites/learn/] pass through C:/websites/learn/javascript/index.php
Nothing is added to Apache or PHP error log; The browser itself receives status code 200, along with the following response headers
Date: "..."
Server: "Apache/2.4.16 (Win32) PHP/5.6.23"
X-Powered-By: "PHP/5.6.23"
Content-Length: "0"
Keep-Alive: "timeout=5, max=100"
Connection: "Keep-Alive"
Content-Type: "text/html; charset=UTF-8"
The response body is an empty string. Like I said, if I rename the file to index.html the content (vanilla html file) is shown. What could be going on?

Answer 1

I figured it out. That was just me being careless. The problem was with this line from Vhosts config:
php_value auto_prepend_file "C:\websites\learn\noop.php"
More specifically, what was supposed to be a noop was really an execution killer.
noop.php
<?php
exit; // <- exits not just this script but all of PHP and returns to the browser
Once I removed the 2nd line, things were back in order. This also explains why renaming index.php to index.html got things working: it took PHP out of the loop entirely.