In what situations can the HTTP_REFERER value be unreliable or manipulated by the user's browser when using PHP?

The HTTP_REFERER value can be unreliable or manipulated by the user's browser when navigating from a secure (HTTPS) site to a non-secure (HTTP) site, or when the user has disabled referrer information in their browser settings. To mitigate this issue, you can check if the HTTP_REFERER value is set and matches the expected domain before using it in your application. 

if (isset($_SERVER['HTTP_REFERER']) && parse_url($_SERVER['HTTP_REFERER'], PHP_URL_HOST) === 'example.com') {
    // Proceed with using the HTTP_REFERER value
} else {
    // Handle the case where the HTTP_REFERER value is unreliable or manipulated
}

Keywords

HTTP_REFERER unreliable manipulated user's browser PHP

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