Get the last modified date of a remote file

You could probably do something like this using curl_getinfo():

<?php
$curl = curl_init('http://www.example.com/filename.txt');

//don't fetch the actual page, you only want headers
curl_setopt($curl, CURLOPT_NOBODY, true);

//stop it from outputting stuff to stdout
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);

// attempt to retrieve the modification date
curl_setopt($curl, CURLOPT_FILETIME, true);

$result = curl_exec($curl);

if ($result === false) {
    die (curl_error($curl)); 
}

$timestamp = curl_getinfo($curl, CURLINFO_FILETIME);
if ($timestamp != -1) { //otherwise unknown
    echo date("Y-m-d H:i:s", $timestamp); //etc
} 

In PHP you can use the native function :get_headers()

<?php
$h = get_headers($url, 1);

$dt = NULL;
if (!($h || strstr($h[0], '200') === FALSE)) {
    $dt = new \DateTime($h['Last-Modified']);//php 5.3
}