Stores PHP code in files with securely hashed names.
The goal of this class is to ensure that if a PHP file is replaced with an untrusted one, it does not get loaded. Since mtime granularity is 1 second, we cannot prevent an attack that happens within one second of the initial save(). However, it is very unlikely for an attacker exploiting an upload or file write vulnerability to also know when a legitimate file is being saved, discover its hash, undo its file permissions, and override the file