Protecting PHP file from direct access. This constant is probably set in the main WordPress PHP file, so if it’s not set, then you aren’t accessing the main page.
Constants aren’t something a user can modify, they are in the code, and can only be changed by editing the PHP file. So, if a hacker could set this constant, then you’d have bigger problems, because that would involve him actually editing your PHP files.
Direct access may not be dangerous (don’t quote me), but it’s pointless. Directly accessing a WordPress plugin (for example) won’t do anything, as the plugin needs to run through WordPress.
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