Version 3 now uses OAuth2 as required by Bitly. Get your developer access token here
Install via composer - In the terminal:
composer require leadthread/php-bitly
use LeadThread\Bitly\Bitly;
$c = new Bitly("access token");
$result = $c->shorten("https://www.google.com/");
var_dump($result);
// string(21) "http://bit.ly/1SvUIo8"