php-etag.php

 <?php 
    $file = 'myfile.php';
    $last_modified_time = filemtime($file); 
    $etag = md5_file($file); 

    header("Last-Modified: ".gmdate("D, d M Y H:i:s", $last_modified_time)." GMT"); 
    header("Etag: $etag"); 

    if (@strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) == $last_modified_time || 
        trim($_SERVER['HTTP_IF_NONE_MATCH']) == $etag) { 
        header("HTTP/1.1 304 Not Modified"); 
    exit; 
} 
?>

function set_cache($content) {
$etag = md5(serialize($content));

header("Etag: $etag");
if ($_SERVER['HTTP_IF_NONE_MATCH'] && trim($_SERVER['HTTP_IF_NONE_MATCH']) == $etag) {
    header("HTTP/1.1 304 Not Modified");
    exit;
}

}