Note also that the URL shown in $HTTP_REFERER is not always the URL of the web page where the user clicked to invoke the PHP script.
This may instead be a document of your own web site, which contains an HTML element whose one attribute references the script. Note also that the current page fragment (#anchor) may be transmitted or not with the URL, depending on the browser.
document.writeln('<FRAME src="your-page-script.php?js=1&ref=' +
document.location + '">');