How to use PHPMailer without composer?

Here's the full working example (though you see a few variables that must be defined and set):

use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\Exception;

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

$mail = new PHPMailer;
$mail->isSMTP(); 
$mail->SMTPDebug = 2; // 0 = off (for production use) - 1 = client messages - 2 = client and server messages
$mail->Host = "smtp.gmail.com"; // use $mail->Host = gethostbyname('smtp.gmail.com'); // if your network does not support SMTP over IPv6
$mail->Port = 587; // TLS only
$mail->SMTPSecure = 'tls'; // ssl is depracated
$mail->SMTPAuth = true;
$mail->Username = $smtpUsername;
$mail->Password = $smtpPassword;
$mail->setFrom($emailFrom, $emailFromName);
$mail->addAddress($emailTo, $emailToName);
$mail->Subject = 'PHPMailer GMail SMTP test';
$mail->msgHTML("test body"); //$mail->msgHTML(file_get_contents('contents.html'), __DIR__); //Read an HTML message body from an external file, convert referenced images to embedded,
$mail->AltBody = 'HTML messaging not supported';
// $mail->addAttachment('images/phpmailer_mini.png'); //Attach an image file

if(!$mail->send()){
    echo "Mailer Error: " . $mail->ErrorInfo;
}else{
    echo "Message sent!";
}

PHP Mailer original source : PHP Mailer In gitHub

// for use PHP Mailer without composer : 

// ex. Create a folder in root/PHPMAILER
// Put on this folder this 3 files find in "src" 
// folder of the distribution : 
// PHPMailer.php , SMTP.php , Exception.php
  
  // include PHP Mailer
  use PHPMailer\PHPMailer\PHPMailer;
  use PHPMailer\PHPMailer\Exception;
  include dirname(__DIR__) .'/PHPMAILER/PHPMailer.php';
  include dirname(__DIR__) .'/PHPMAILER/SMTP.php';
  include dirname(__DIR__) .'/PHPMAILER/Exception.php';

  // i made a function 

   /*
    *
    * Function send_mail_by_PHPMailer($to, $from, $subject, $message);
    * send a mail by PHPMailer method
    * @Param $to -> mail to send
    * @Param $from -> sender of mail
    * @Param $subject -> suject of mail
    * @Param $message -> html content with datas
    * @Return true if success / Json encoded error message if error 
    * !! need -> classes/Exception.php - classes/PHPMailer.php - classes/SMTP.php
    *
    */
    function send_mail_by_PHPMailer($to, $from, $subject, $message){

          // SEND MAIL by PHP MAILER
          $mail = new PHPMailer();
          $mail->CharSet = 'UTF-8';
          $mail->isSMTP(); // Use SMTP protocol
          $mail->Host = 'your_host.com'; // Specify  SMTP server
          $mail->SMTPAuth = true; // Auth. SMTP
          $mail->Username = 'my_mail@your_host.com'; // Mail who send by PHPMailer
          $mail->Password = 'your_passord_of_your_box'; // your pass mail box
          $mail->SMTPSecure = 'ssl'; // Accept SSL
          $mail->Port = 465; // port of your out server
          $mail->setFrom($from); // Mail to send at
          $mail->addAddress($to); // Add sender
          $mail->addReplyTo($from); // Adress to reply
          $mail->isHTML(true); // use HTML message
          $mail->Subject = $subject;
          $mail->Body = $message;

          // SEND
          if( !$mail->send() ){

              // render error if it is
              $tab = array('error' => 'Mailer Error: '.$mail->ErrorInfo );
              echo json_encode($tab);
              exit;
          }
          else{
              // return true if message is send
              return true;
          }

    }
    /*
    *
    * END send_mail_by_PHPMailer($to, $from, $subject, $message)
    * send a mail by PHPMailer method
    *
    */
  
    // use function :
    send_mail_by_PHPMailer($to, $from, $subject, $message);

This worked for me, I also downloaded phpmailer from this address

https://sourceforge.net/projects/phpmailer/

use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\Exception;
use PHPMailer\PHPMailer\SMTP;

require '/source/PHPMailer2/src/Exception.php';
require '/source/PHPMailer2/src/PHPMailer.php';
require '/source/PHPMailer2/src/SMTP.php';