php composer dependency internal server error

I installed a dependency (Minishlink Web-Push) via composer2 on my server. The installation returned a success and in my homepages file manager I find the files composer.json and composer.lock now.

The files show the dependency I want to use.

However, sending an Ajax POST request to my php file returns an internal server error as soon as I enter a piece of code that is trying to access the dependency. I tried several ways to access the dependency, but it does not work.

Internal server Error:

POST <MY_URL> 500 (Internal Server Error)

My php file, accessed with an Ajax POST request:

if ($_SERVER["REQUEST_METHOD"] == "POST") {


    // probably needed to initialize an instance of my dependency
    $auth = [
        'VAPID' => [
            'subject' => 'https://<MY_DOMAIN>',
            'publicKey' => '<SERVER_PUBLIC_KEY>',
            'privateKey' => '<SERVER_PRIVATE_KEY>'
        ]
    ];



    // I tried all eight options below:

    $webPush = new MinishlinkWebPushWebPush($auth);
    //ERROR: PHP Fatal error:  Uncaught Error: Class "MinishlinkWebPushWebPush" not found in /home/<USER>/domains/<MY_DOMAIN>/public_html/push/subscription.php

    $webPush = new MinishlinkWebPushWebPush($auth);
    //ERROR: PHP Fatal error:  Uncaught Error: Class "MinishlinkWebPushWebPush" not found in /home/<USER>/domains/<MY_DOMAIN>/public_html/push/subscription.php

    $webPush = new /Minishlink/WebPush/WebPush($auth);

    $webPush = new Minishlink/WebPush/WebPush($auth);


    use MinishlinkWebPushWebPush;
    $webPush = new WebPush($auth);
    //ERROR: PHP Parse error:  syntax error, unexpected token "use" in /home/<USER>/domains/<MY_DOMAIN>/public_html/push/subscription.php

    use MinishlinkWebPushWebPush;
    $webPush = new WebPush($auth);
    //ERROR: PHP Parse error:  syntax error, unexpected token "use" in /home/<USER>/domains/<MY_DOMAIN>/public_html/push/subscription.php

    use /Minishlink/WebPush/WebPush;
    $webPush = new WebPush($auth);
    //ERROR: PHP Parse error:  syntax error, unexpected token "use" in /home/<USER>/domains/<MY_DOMAIN>/public_html/push/subscription.php

    use Minishlink/WebPush/WebPush;
    $webPush = new WebPush($auth);
    //ERROR: PHP Parse error:  syntax error, unexpected token "use" in /home/<USER>/domains/<MY_DOMAIN>/public_html/push/subscription.php




    // general success variable
    $success = true;


    // build response object
    $response = (object) ['success' => $success, 'data' => (object) []];


    // return response
    echo json_encode($response);


    //--> without dependency it returns {success: true, data: {}}

}