how to upload a file with shopify's file api

So just to clarify what I am trying to do.

I do NOT want to upload an image to a product.

I want to use the File API to upload a file to files

I don’t know graphql very well at all.

I have setup a private app, and set the file scopes for the Admin API.
I have copied the Admin API access token, stored as SHOPIFY_ACCESS_TOKEN

I am using shopify’s php sdk

Context::initialize(
        env('SHOPIFY_API_KEY'),
        env('SHOPIFY_API_SECRET'),
        env('SHOPIFY_APP_SCOPES'),
        env('SHOPIFY_DOMAIN'),
        new FileSessionStorage('/tmp/php_sessions'),
        env('SHOPIFY_API_VERSION'),
        false,
        true,
    );


    $client = new Graphql(env("SHOPIFY_DOMAIN"), env("SHOPIFY_ACCESS_TOKEN"));

    $query = <<<QUERY
  mutation fileCreate($files: [FileCreateInput!]!) {
    fileCreate(files: $files) {
      files {
        alt
        createdAt
      }
    }
  }
QUERY;
    $variables = [
        "files" => [
            "alt" => "this is a cat",
            "contentType" => "IMAGE",
            "originalSource" => "https://placekitten.com/1300/1300",
        ],
    ];

    $response = $client->query(["query" => $query, "variables" => $variables]);

The Query part i copied directly from the FileCreate docs.

The Context::initialize I got from the sdk docs.

The error code that is returning is
401{"errors":"[API] Invalid API key or access token (unrecognized login or wrong password)"}

I am unsure where to go from here and how to get it working.

Can anyone see where I may have gone wrong?

Edit: 20231112

A huge thanks to @hamzasgd for the awesome info.

I have managed to get it working with PHP, which is great.

My issue auth issue was that I was passing the store front api key, instead of the admin api key into the Context::initialize method.

It should have been this

    Context::initialize(
        env('SHOPIFY_API_KEY'),
        env('SHOPIFY_ACCESS_TOKEN'),
        env('SHOPIFY_APP_SCOPES'),
        env('SHOPIFY_DOMAIN'),
        new FileSessionStorage('/tmp/php_sessions'),
        ApiVersion::LATEST,
        false,
        true,
    );