PHPMailer Syntax Error Fresh Install

Question

Using the following setup;

• PHP version: 5.3.3
• PHPMailer version: 6.0.5
• CentOS release: 6.9

I manually downloaded the PHPMailer ZIP from the github source, extracted the files onto my local Win 10 PC, then FTP'd them to a remote folder on the CentOS machine (var/www/html/mysite/lib/PHPMailer).

I created the file mysite/mailer.php and the contents are identical to the example on github (path names changed to reflect my config) - see below;

<?php
use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\Exception;

require 'lib/PHPMailer/src/Exception.php';
require 'lib/PHPMailer/src/PHPMailer.php';
require 'lib/PHPMailer/src/SMTP.php';

$mail = new PHPMailer(true);                              // Passing `true` enables exceptions
try {
    //Server settings
    $mail->SMTPDebug = 2;                                 // Enable verbose debug output
    $mail->isSMTP();                                      // Set mailer to use SMTP
    $mail->Host = 'smtp1.example.com;smtp2.example.com';  // Specify main and backup SMTP servers
    $mail->SMTPAuth = true;                               // Enable SMTP authentication
    $mail->Username = 'user@example.com';                 // SMTP username
    $mail->Password = 'secret';                           // SMTP password
    $mail->SMTPSecure = 'tls';                            // Enable TLS encryption, `ssl` also accepted
    $mail->Port = 587;                                    // TCP port to connect to

    //Recipients
    $mail->setFrom('from@example.com', 'Mailer');
    $mail->addAddress('joe@example.net', 'Joe User');     // Add a recipient
    $mail->addAddress('ellen@example.com');               // Name is optional
    $mail->addReplyTo('info@example.com', 'Information');
    $mail->addCC('cc@example.com');
    $mail->addBCC('bcc@example.com');

    //Attachments
    $mail->addAttachment('/var/tmp/file.tar.gz');         // Add attachments
    $mail->addAttachment('/tmp/image.jpg', 'new.jpg');    // Optional name

    //Content
    $mail->isHTML(true);                                  // Set email format to HTML
    $mail->Subject = 'Here is the subject';
    $mail->Body    = 'This is the HTML message body <b>in bold!</b>';
    $mail->AltBody = 'This is the body in plain text for non-HTML mail clients';

    $mail->send();
    echo 'Message has been sent';
} catch (Exception $e) {
    echo 'Message could not be sent. Mailer Error: ', $mail->ErrorInfo;
}

?>

Without changing any of the config settings, when I visit mydomain.com/mailer.php I see the following error;

Parse error: syntax error, unexpected '[' in /var/www/html/mysite/lib/PHPMailer/src/PHPMailer.php on line 304

Line 304 of PHPMailer.php contains;

public $SMTPOptions = [];

I've tried entering my own SMTP details in the mailer.php file but I still receive the same error.

Any ideas what the issue is?

Any help is appreciated.