error in contactus form after hosting it on server

Question

I am facing this error. I have created one contacts form in php via which I can receive mails from users. I uploaded it on big rock hosting. But I am getting this error which is in the image I am giving my php code in description.

<?php
    $name = $_POST['name'];
    $from = $_POST['email'];
    $phone = $_POST['phone']; 
    $email = "[email protected]" ;
    $message = $_POST['message'];
    $subject = $name+$phone;
    mail($email,$subject,$message,"From".$from);
    echo "mail sent";
    header('Location: index.php');
?>`