Does a remote file exist (using get_headers)?

As you probably know, a get_headers function in PHP typically returns something similar to:

Array
(
    [0] => HTTP/1.1 200 OK
    [1] => Content-Type: image/png
    [2] => Content-Length: 127999
    [3] => Connection: close
    [4] => Date: Thu, 09 Nov 2017 12:16:51 GMT
    [5] => Last-Modified: Wed, 08 Nov 2017 09:12:17 GMT
    ...
)

Where the first element of the array contains a response status. With that in mind, let’s use it for our needs.

<?php
function remote_file_exists($url) {
    $file_headers = get_headers($url);

    $not_found_headers = [
        'HTTP/1.1 404 Not Found',
        'HTTP/1.0 404 Not Found',
    ];

    return !in_array($file_headers[0], $not_found_headers);
}

$url = 'https://upload.wikimedia.org/wikipedia/commons/thumb/2/27/PHP-logo.svg/100px-PHP-logo.svg.png';
$res = remote_file_exists($url);
var_dump($res); // Output: bool(true)

// ... and use a fake URL
$url = 'https://upload.wikimedia.org/wikipedia/commons/thumb/2/27/PHP-logo.svg/100px_FAKE-PHP-logo.svg.png';
$res = remote_file_exists($url);
var_dump($res); // Output: bool(false)

The first thing to notice is that the response message might have the different version of the protocol: HTTP/1.0 404 Not Found, HTTP/1.1 404 Not Found etc.

And the second one - get_headers() won’t work properly in case allow_url_fopen is disabled.

To check if allow_url_fopen is enabled or not, you can use ini_get():

<?php
if (ini_get('allow_url_fopen')) {
    echo 'allow_url_fopen is enabled';
}

If it’s possible edit your php.ini file, find allow_url_fopen and specify

allow_url_fopen=1

In case you don’t have access to do this, use the following code in your PHP script (from the very beginning):

<?php 
ini_set('allow_url_fopen', 1);