Update Spotify.php

getTracksInPlaylist returns a maximum of 100 results. If there are more than 100 results, then you need to specify an offset of the previous offset + 100. Something I have done to mine, which could possibly done here, is to automatically return all results.

$data['next'] = 'start';
while($data['next'] != '')
{			
    $data = $this->request('GET', SPOTIFY_API_ENDPOINT, '/users/' . $user . '/playlists/' . $playlist . '/tracks?      offset=' . $offset , 'json', NULL);
    $offset = $offset + 100;
    foreach($data['items'] as $item)
    {
        $collection[] = $item;
    }
}
return $collection[];