Accessing JSON with PHP leads to no result ("Trying to access array offset on value of type null")

I want to access an API-link (JSON) but my PHP script strangely does not seem to find the correct values. It returns a NULL instead.

It cannot find $obj['message']['total-results'] even though I'm sure it exists (and when I open the API-Link manually in the browser, I do see it).

With the following code, therefore ...

<?php
# display errors
ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);

# curl function
# found via https://stackoverflow.com/questions/8540800/how-to-use-curl-instead-of-file-get-contents
function file_get_contents_curl($url) {
    $ch = curl_init();

    curl_setopt($ch, CURLOPT_AUTOREFERER, TRUE);
    curl_setopt($ch, CURLOPT_HEADER, 0);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_FOLLOWLOCATION, TRUE);   
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);

    $data = curl_exec($ch);
    curl_close($ch);

    return $data;
}

$apiurl = "https://api.crossref.org/journals/1389-5575/works?query=since-pub-date:2019-09-22&select=is-referenced-by-count"

# get JSON contents
$obj = file_get_contents_curl($apiurl);
$obj = json_decode($obj, true);

# handle error
if($obj == "Resource not found.") {

  echo 'API connection failed';

} else {

  echo 'API connection succeeded';

  var_dump($obj); # NULL

  if(!isset($obj['message']['total-results'])) {
            
    echo '<br />ERROR: No $obj-message-total-results found!';

  }

}

$new = $obj['message']['total-results']; // Warning: Trying to access array offset on value of type null ...

?>

... I get:

• "API connection succeeded"
• "ERROR: No $obj-message-total-results found!"
• "Warning: Trying to access array offset on value of type null"

Did I get anything wrong? I've been using that API for many years without any problems, so I'm really unsure what's the matter here. Thanks for your help!