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Short answer: it's safe. Longer answer: it's as safe as any other PHP file on your server. It's a common practice to put this script, or at least a script that defines variables/constants with database credentials, in a PHP file that is not located inside the web root (eg, outside of your public_html or www or whatever directory that your site is based in) because if it's not an actual page then it really shouldn't be in the root; this practice is easy to achieve when you get larger sites that have a single public_html/index.php that runs an "application" or some similar concept whose files are all outside the root.