How do I set up caching headers, or am I set to a short time?

Question

caching

I still have a problem with caching
, I have code on each page

<?php
$LastModified_unix = strtotime(date("D, d M Y H:i:s", filectime($_SERVER['SCRIPT_FILENAME'])));
$LastModified = gmdate("D, d M Y H:i:s \G\M\T", $LastModified_unix);
$IfModifiedSince = false;
if (isset($_ENV['HTTP_IF_MODIFIED_SINCE']))
    $IfModifiedSince = strtotime(substr($_ENV['HTTP_IF_MODIFIED_SINCE'], 5));  
if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']))
    $IfModifiedSince = strtotime(substr($_SERVER['HTTP_IF_MODIFIED_SINCE'], 5));
if ($IfModifiedSince && $IfModifiedSince >= $LastModified_unix) {
    header($_SERVER['SERVER_PROTOCOL'] . ' 304 Not Modified');
    exit;
}
header('Last-Modified: '. $LastModified);
?>

when checking the page, it writes such information
"There are no caching headers in the server response or it is configured for too short a time"
how to set it up correctly, please tell me.