Libraries API not working in my module
Add a composer.json file to your movies module &amp; name it your custom module's name:

{
  "name": "drupal/movies",
  "type": "drupal-module",
  "license": "GPL-2.0+",
  "require": {
    "imdbphp/imdbphp": "*"
  }
}

Add a new line to the merge-plugin include array in the composer.json of your Drupal ROOT with your module's composer file path:

"extra": {
  "merge-plugin": {
    "include": [
        "core/composer.json",
        "modules/custom/movies/composer.json"
    ]
  }
}

Enable the movies module
Run composer update &amp; composer dumpauto in Drupal ROOT. This will install &amp; autoload the new dependencies in your vendor directory.
Now you can use the classes provided by this library. In your case for example:

$title = new \Imdb\Title(335266);