I am facing a strange issue with WordPress. When I put this line as a code snippet in a blog post:
$app = require_once __DIR__.'/../bootstrap/app.php';
I get a 404 error when I try to preview the page, and I wont be able to save the post at all but if I make the line of code incorrect for example, remove the last 'e' from require, this way:
$app = requir_once __DIR__.'/../bootstrap/app.php';
everything works fine. Is the code somehow executing and causing problems?
Your web host probably has a paranoid web application firewall (most likely mod_security) enabled, with rules to block requests which contain data which looks like PHP code.
Contact your web host and request that they turn this feature off.
Wordpress is a little funny that way, try using get_stylesheet_directory() function. This will return the path to your theme directory.
require_once( get_stylesheet_directory() . '/path/to/bootstrap.css );
Also the require_once function doesn't return anything so there is no point trying to assign it to a variable.