So I've found a ton of Goo.gl API shortener classes out there but nothing worked for me (maybe because of wrong cURL options or outdated urls, etc). This one is simple and works nicely. The function doesn't need any API keys and returns the original url if curl isn't present.

BFIGoogleUrl.php

class BFIGoogleUrl {
    public static function shorten($url) {
        if (!function_exists('curl_init')) 
            return $url;
            
        $api = 'https://www.googleapis.com/urlshortener/v1/url?';
        $data_string = '{"longUrl":"'.$url.'"}';
        
        # Initialize cURL
        $ch = curl_init();
        # Set our default target URL
        curl_setopt($ch, CURLOPT_URL, $api);
        # We don't want the return data to be directly outputted, so set RETURNTRANSFER to true
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        # Set cURL options
        curl_setopt($ch, CURLOPT_POST, 1);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);
        curl_setopt($ch, CURLOPT_HTTPHEADER, Array('Content-Type: application/json'));
        curl_setopt($ch, CURLOPT_TIMEOUT, CURL_TIMEOUT);
        curl_setopt($ch, CURLOPT_USERAGENT, "Mozilla/5.0 (Windows NT 6.1) AppleWebKit/534.30 (KHTML, like Gecko) Chrome/12.0.742.122 Safari/534.30"); // from timthumb
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
        
        // get the shortened url
        $ret = json_decode(curl_exec($ch))->id;
        
        # Close the curl handle
        curl_close($ch);
        $ch = null;
        
        return $ret;
    }
}