Return to Revisions

In my initial problem posted on SO, whenever anyone accessed the Mail.php on the server, it used to send an empty email to the $to. To avoid this I came up with a solution.

require 'PHPMailer/PHPMailerAutoload.php';
 
 $yourName = $_POST['yourName'];
 $sender = $_POST['emailID'];
 $subject = $_POST['subject'];
 $message = $_POST['message'];
 $to = '[email protected]';
 if(empty($yourName) || empty($sender) || empty($subject) || empty($message) || empty($message))
 {
 echo "Fields are empty";
 }
 else
 {
 $mail = new PHPMailer;
 //$mail->SMTPDebug = 2; 
 $mail->isSMTP(); 
 $mail->Host = 'smtp.gmail.com'; 
 $mail->SMTPAuth = true; 
 $mail->Username = 'gmailUser'; 
 $mail->Password = 'gmailPassword'; 
 $mail->SMTPSecure = 'tls'; 
 $mail->Port = 587; 
 $mail->setFrom($sender,$yourName);
 $mail->addAddress($to); 
 $mail->addReplyTo($sender);
 $mail->isHTML(true); 
 $mail->Subject = $subject;
 $mail->Body = "<b>From: </b>". $sender. "<br>" ." <b>Name: </b>". $yourName. "<br>". "<b> Message Body </b>" .$message;
 $mail->AltBody = "<b>From: </b>". $sender. "<br>" ." <b>Name: </b>". $yourName. "<br>". "<b> Message Body </b>" .$message;
 if(!$mail->send()) 
 {
 echo 'Message could not be sent.';
 echo 'Mailer Error: ' . $mail->ErrorInfo;
 } 
 else 
 {
 echo "Message has been sent....You're being redirected.....";
 }
 }

This fix, basically allows the user to interact with Mail.php, but Mail.php doesn't sends a null email to $to

Now hours later, I find out that this is a very bad approach.

I would like to know how a professional would solve this problem?

Any good approach which I could use to efficiently optimize the code?