whm settings for php mail

You can't do that because PHPMailer can use at least 3 different methods of sending email.  Or do you mean the PHP mail() function?  Those are two different things although PHPMailer (which is a PHP program you can download) can use the PHP mail() function also.  It can also send directly to an external SMTP server and I don't remember what the third method is.  Direct socket connection I believe.

On the WHM mail options for PHP, I think there is a way that only lets the server send email if I use an email account , ( username and password ) otherwise it shouldn't send email

The web site contact forms on my server should only send emails using the below code

<?php
require 'class.phpmailer.php';

$mail = new PHPMailer;

$mail->IsSMTP();                                      // Set mailer to use SMTP
$mail->Host = 'smtp1.example.com;smtp2.example.com';  // Specify main and backup server
$mail->SMTPAuth = true;                               // Enable SMTP authentication
$mail->Username = 'jswan';                            // SMTP username
$mail->Password = 'secret';                           // SMTP password
$mail->SMTPSecure = 'tls';                            // Enable encryption, 'ssl' also accepted

$mail->From = 'from@example.com';
$mail->FromName = 'Mailer';
$mail->AddAddress('josh@example.net', 'Josh Adams');  // 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');

$mail->WordWrap = 50;                                 // Set word wrap to 50 characters
$mail->AddAttachment('/var/tmp/file.tar.gz');         // Add attachments
$mail->AddAttachment('/tmp/image.jpg', 'new.jpg');    // Optional name
$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';

if(!$mail->Send()) {
   echo 'Message could not be sent.';
   echo 'Mailer Error: ' . $mail->ErrorInfo;
   exit;
}

echo 'Message has been sent';

You can restrict the local mail server but 'class.phpmailer.php' includes all the methods that I was talking about.  The SMTP method and the socket method totally bypass the mail server on your site.

is my web site safe considering php mails if I can use above code to send email on my contact form? is it possible to any body to force my server send spam emails?