使用带有codeigniter的mailgun API

I'm trying to use the mailgun api to send user registration confirmation emails from my website. My site is on a shared host. The mailgun api seems to work perfectly, when I test it out on a localhost (WAMP server). when I try to use the api in my source code, i.e. inside a controller, I get a syntax error. The shared host is a linux based (CentOS) server.

This is the error message.

Parse error: syntax error, unexpected '[' in /var/www/html/log2pdf/mailgun/vendor/guzzlehttp/psr7/src/functions.php on line 78

Since it was a syntax error, I downloaded and installed the mailgun dependancies (i.e. composer, guzzle adapter etc.) separately on the linux server, because I had originally copied the files from my localhost (windows) and was under the impression that these files are different for windows and linux. But that made no difference either.

Running php 5.6.25 on localhost (WAMP), php 5.3.3 on shared host(CentOS). All the mailgun dependencies were installed using the commands on the mailgun webpage.

I have tried all that I could, any help on this will be greatly appreciated.

This is my controller:

<?php
defined('BASEPATH') OR exit('No direct script access allowed');
require '/var/www/html/mysite/mailgun/vendor/autoload.php';
use Mailgun\Mailgun;

class Register extends CI_Controller {

public function __construct()
{
    parent::__construct();
    $this->load->model('User_model');
    $this->load->helper('cookie');
}
public function index($param = '')
{

    if (($this->session->userdata('userLogin') == TRUE))  
    { 
        redirect(site_url('users/dashboard'));  }

    if (isset($_POST['registration'])){

        $data['postdata'] = $_POST;

        if(!empty($_POST['firstname']) && !empty($_POST['lastname']) && 
        !empty($_POST['email']) && !empty($_POST['password']) && 
        !empty($_POST['organization']) && !empty($_POST['country']) && 
        !empty($_POST['address'])){ 

            $checkEmail = $this->User_model->checkEmail();

            if($checkEmail==true){

                $this->session->set_flashdata('error', '<p><font>
        This Email id is already registered!</font></p>');

            }else{

                $un = str_replace(' ', '',$_POST['firstname'].$_POST['lastname']);
                //remove all spaces
                $unwsc  =  preg_replace('/[^A-Za-z0-9\-]/', '', $un); 
                // Removes special chars.

                $checkusername = $this->User_model->checkusername($unwsc);

                if($checkusername==true){

                    $username = $this->getUserName($unwsc);

                }else{
                    $username = $unwsc;
                }

                $data['code'] = rand(0,100000); 
                $data['username'] = $_POST['firstname'];

                $this->User_model->saveUser($username,$data['code']);

                mkdir("/var/www/html/users/".$username);

                /*
                $fromemail="_my_organisation's_email_";
                $subject = "Registration Confirmation";

                //this was the email sender that i used initially
                            //it's codeigniter's built in email library. but after a
                        //while, emails stopped being delivered.

                $this->load->library('email');  
                $config['mailtype'] = 'html'; 
                $this->email->initialize($config);
                $this->email->to($_POST['email']);
                $this->email->from($fromemail, "_my_organisation's_email_");
                $this->email->subject($subject);                    
                $body = $this->load->view('email/register.php',$data,TRUE);
                $this->email->message($body);                   
                $this->email->send();
                */                                      

                $subject = "Registration Confirmation";
                $body = $this->load->view('email/register.php',$data,TRUE);

                //using mailgun api                                 

                # Instantiate the client.
                $mgClient = new Mailgun('_my_mailgun_key_');
                $domain = "_my_maigun_domain_";

                # Make the call to the client.
                $result = $mgClient->sendMessage($domain, array(
                'from'    => 'NO REPLY <_my_organisation's_email_>',
                'to'      => 'new user <_authorized_recipient_>',
                'subject' => 'Mailgun test',
                'text'    => 'testing email sender'
                ));


                $this->session->set_flashdata('success','<p><font>Thanksfor registration!                   </font></p>');
                redirect(site_url('register/complete'));    
            }

        }else{
            $this->session->set_flashdata('error', '<p><font>
            Missing Some Fields!</font></p>');
        }
    }   
    $data['title']='';
    $data['param']=$param;
    $this->load->view('registration/index',$data);
}
function getUserName($string) {
    $result = $string;
    for ($i = 1; $i < 100; $i++) {
         $userChecking = $this->User_model->checkusername($string.$i);
         if(empty($userChecking)){
            $result = $string.$i;
            break;
         }
    }
    return $result;
}
}
?>