Your eyes are fine. You need to check for If-Modified-Since header, if the time is older than file modification time (filemtime()) send Last-Modified header and the image, else send 304 Not Modified response.

This code seems to work. Have I got it right?

// Get the time the cache file was last modified
$lastModified = filemtime($pPath);

// Issue an HTTP last modified header
header('Last-Modified: ' . gmdate('D, d M Y H:i:s', $lastModified) . ' GMT');


if (isset($_GET['If-Modified-Since']))
{
        // Split the If-Modified-Since (Netscape < v6 gets this wrong)
        $modifiedSince = explode(';', $_GET['If-Modified-Since']);
        
        // Turn the client request If-Modified-Since into a timestamp
        $modifiedSince = strtotime($modifiedSince[0]);
}
else
{
        $modifiedSince = 0;
}

// Compare time the content was last modified with client cache
if ($lastModified <= $modifiedSince)
{
        header('HTTP/1.1 304 Not Modified');
}
else
{
        $extention = substr($path, -3);
        if ($extention == "jpg")
                header("Content-type: image/jpeg");
        if ($extention == "gif")
                header("Content-type: image/gif");
        if ($extention == "bmp")
                header("Content-type: image/bmp");
        if ($extention == "png")
                header("Content-type: image/png");
        readfile("$pPath");
}

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