Git Deployment PHP class

GitDeploy.php

<?php
// Set your local timezone - http://www.php.net/manual/en/timezones.php
date_default_timezone_set('Europe/London'); 

/**
 * Git Repo Deploment class.
 * @version 1-2
 * @author (original) arindambiswas <http://www.collectivezen.com>
 * @link https://gist.github.com/arindambiswas/4634fff09b3ad4e4bf48
 * 
 * @version 3
 * @author lcherone <http://iCodes.it>
 * @link https://gist.github.com/lcherone/e2d48c8f5fbfec7b3da7
 * 
 */
class Deploy {

    /**
     * The name of the file that will be used for logging deployments. Set to 
     * FALSE to disable logging.
     * 
     * @var string
     */
    private $_log = './deploy.log';

    /**
     * The timestamp format used for logging.
     * 
     * @link    http://www.php.net/manual/en/function.date.php
     * @var     string
     */
    private $_date_format = 'Y-m-d H:i:sP';

    /**
     * The name of the branch to pull from.
     * 
     * @var string
     */
    private $_branch = 'master';

    /**
     * The name of the remote to pull from.
     * 
     * @var string
     */
    private $_remote = 'origin';

    /**
     * The directory where your website and git repository are located, can be 
     * a relative or absolute path
     * 
     * @var string
     */
    private $_directory;
    private $_cwd;

    /**
     * Sets up defaults.
     * 
     * @param  string  $directory  Directory where your website is located
     * @param  array   $data       Information about the deployment
     */
    public function __construct($directory, $options = array())
    {
        // determine the directory paths
        $this->_directory = realpath($directory).DIRECTORY_SEPARATOR;
        $this->_cwd       = realpath(getcwd()).DIRECTORY_SEPARATOR;
    }

    /**
     * Writes a message to the log file.
     * 
     * @param  string  $message  The message to write
     */
    public function log($message)
    {
        // change to scripts working directory 
        chdir($this->_cwd);
        
        // echo the log message
        echo $message.'<br>';
        
        // log it to file if enabled
        if ($this->_log != false) {

            if (!file_exists($this->_log)) {
                // create the log file
                file_put_contents($this->_log, '');

                // allow anyone to write to log files
                chmod($this->_log, 0666);
            }

            // write the message into the log file
            // format: time --- type: message
            file_put_contents($this->_log, 
                date($this->_date_format).' : '.$message.PHP_EOL, FILE_APPEND);
        }

        // change back to repos working directory
        chdir($this->_directory);
    }

    /**
     * Executes the necessary commands to deploy the website.
     */
    public function execute()
    {
        try {
            $this->log('Attempting deployment.');
            
            // make sure we're in the right directory
            chdir($this->_directory);
            $this->log('Changing working directory.');

            
            // discard any changes to tracked files since our last deploy
            exec('git reset --hard HEAD', $output);
            if(empty($output)) 
                throw new Exception ('Error resetting local repo HEAD');
            $this->log('Reseting repository: '.implode(' ', $output));

            // update the local repository
            exec('git pull '.$this->_remote.' '.$this->_branch, $output);
            if(empty($output[1])) 
                throw new Exception ('Error pulling in changes');
            $this->log('Pulling in changes: '.implode(' ', $output));

            // secure the .git directory
            exec('chmod -R og-rx .git', $output);
            $this->log('Securing .git directory.');

            $this->log('Deployment successful!');
        
        }catch(Exception $e) {
            $this->log($e->getMessage());
        }
    }

}

/**
 * The root directory where the repos live, relative to script or use absolute.
 * 
 * @var string
 */
$root_dir = '../Git_Projects/';

/**
 * Name of the repo to pull
 * 
 * @var string
 */
$repo = 'tridentCMS';

/**
 * Go deployment!
 */
$deploy = new Deploy($root_dir.$repo);
$deploy->execute();
?>