What are the potential reasons for inconsistent logging of referrers in PHP when using an img tag?

The inconsistent logging of referrers in PHP when using an img tag may be due to browser settings, privacy concerns, or the referrer being stripped by proxies. To ensure consistent logging of referrers, you can set a default referrer when it is not provided by the browser. This can be done by checking if the referrer is empty and then setting a default referrer value.

// Check if referrer is empty
if(empty($_SERVER['HTTP_REFERER'])) {
    // Set a default referrer value
    $_SERVER['HTTP_REFERER'] = 'https://example.com/default-referrer';
}

// Log the referrer
$referrer = $_SERVER['HTTP_REFERER'];
error_log('Referrer: ' . $referrer);