you have four fields to check. Its 4 IF conditions to check if they have been filled.
If you want to make it quicker, build function(s), like:
<?php
function checkkey($index , $label, $required)
{
if ($required == 1 AND empty($_POST[$index]))
return "$label is required<br />";
}
$errors = '';
$errors .= checkkey("name" , "Name", 1);
$errors .= checkkey("email" , "E-mail address", 1);
$errors .= checkkey("subject" , "Subject", 0);
$errors .= checkkey("message" , "Your message", 0);
if ($errors != '')
echo $errors;
else {
// send email
}
?>