Try taking a look at http://code.google.com/p/gtranslate-api-php/ which claims to be able to do this. Please note that a required file (GTranslate.php) currently only seems to be available in the SVN repository of this project.

Want to use Google Translate API with PHP (server-side), you would need to insert "google/cloud-translate" and "google/apiclient" in your composer file, run composer update and call translate method after creating an object of the following class:

<?php

require_once APP_ROOT .  /library/google-api-php-client/src/Google_Client.php ;
require_once APP_ROOT .  /library/google-api-php-client/src/contrib/Google_TranslateService.php ;

class GoogleTranslator
{
    private $developer_key;

    const PROVIDER = "google";
    const SUCCESS = "success";
    const FAILURE = "failure";
    const TRANSLATE_SUCCESS_MESSAGE =  text translation successful ;

    public function __construct()
    {
        $this->developer_key =  <DEVELOPER KEY HERE> ;
    }

    public function translate($text, $source_language, $target_language)
    {
        try {
            $client = new Google_Client();
            $client->setApplicationName( Google Translate PHP Starter Application );

            $client->setDeveloperKey($this->developer_key);
            $service = new Google_TranslateService($client);

            $optional_parameters = array( source  => $source_language);

            $translated_text = $service->translations->listTranslations($text, $target_language, $optional_parameters);
            return array( translation  => $translated_text[ translations ][0][ translatedText ],
                 provider  => self::PROVIDER,
                 status  => self::SUCCESS,
                 message  => self::TRANSLATE_SUCCESS_MESSAGE);
        } catch (Exception $e) {
            error_log($e->getMessage() . " FOR GET TRANSLATION CALL", "GOOGLE TRANSLATE API");
            $res = array( translation  =>   ,
                 provider  => self::PROVIDER,
                 status  => self::FAILURE,
                 message  => $e->getMessage());
            return $res;
        }
    }
}