为什么我的PHP联系表格不起作用？

Question

<?php
 $to = "myemail@gmail.com";
 $email = $_REQUEST['email'] ;
 $username = $_REQUEST['username'] ;
 $subject = "Application from" . $username;
 $headers = "From:" . $email;
 $reason = $_REQUEST['reason'] ;
 $sea = $_REQUEST['plant'] ;
 $body = $reason . "<br/>" . $sea ;
 if (mail($to, $subject, $body))
   print "Application sent. We will get back to you soon." ;
 else
   print "Something went wrong" ;
?>

它总是会出问题出错并且不起作用。我做错了什么？