<?php
if(isset($_POST['email'])) {
  
  // EDIT THE 2 LINES BELOW AS REQUIRED
  $email_to = "you@yourdomain.com";
  $email_subject = "Your email subject line";
  
  
  function died($error) {
    // your error code can go here
    echo "We are very sorry, but there were error(s) found with the form you submitted. ";
    echo "These errors appear below.<br /><br />";
    echo $error."<br /><br />";
    echo "Please go back and fix these errors.<br /><br />";
    die();
  }
  
  // validation expected data exists
  if(!isset($_POST['first_name']) ||
    !isset($_POST['last_name']) ||
    !isset($_POST['email']) ||
    !isset($_POST['telephone']) ||
    !isset($_POST['comments'])) {
    died('We are sorry, but there appears to be a problem with the form you submitted.');    
  }
  
  $first_name = $_POST['first_name']; // required
  $last_name = $_POST['last_name']; // required
  $email_from = $_POST['email']; // required
  $telephone = $_POST['telephone']; // not required
  $comments = $_POST['comments']; // required
  
  $error_message = "";
  $email_exp = '/^[A-Za-z0-9._%-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}$/';
  if(!preg_match($email_exp,$email_from)) {
    $error_message .= 'The Email Address you entered does not appear to be valid.<br />';
  }
  $string_exp = "/^[A-Za-z .'-]+$/";
  if(!preg_match($string_exp,$first_name)) {
    $error_message .= 'The First Name you entered does not appear to be valid.<br />';
  }
  if(!preg_match($string_exp,$last_name)) {
    $error_message .= 'The Last Name you entered does not appear to be valid.<br />';
  }
  if(strlen($comments) < 2) {
    $error_message .= 'The Comments you entered do not appear to be valid.<br />';
  }
  if(strlen($error_message) > 0) {
    died($error_message);
  }
  $email_message = "Form details below.\n\n";
  
  function clean_string($string) {
    $bad = array("content-type","bcc:","to:","cc:","href");
    return str_replace($bad,"",$string);
  }
  
  $email_message .= "First Name: ".clean_string($first_name)."\n";
  $email_message .= "Last Name: ".clean_string($last_name)."\n";
  $email_message .= "Email: ".clean_string($email_from)."\n";
  $email_message .= "Telephone: ".clean_string($telephone)."\n";
  $email_message .= "Comments: ".clean_string($comments)."\n";
  
  
// create email headers
$headers = 'From: '.$email_from."\r\n".
'Reply-To: '.$email_from."\r\n" .
'X-Mailer: PHP/' . phpversion();
@mail($email_to, $email_subject, $email_message, $headers);  
?>
 
<!-- include your own success html here -->
 
Thank you for contacting us. We will be in touch with you very soon.
 
<?php
}
?>

And here is a contact form you can use to test the form2mail program:

<form name="contactform" method="post" action="form2mail.php">
<table width="450">
</tr>
<tr>
 <td valign="top">
  <label for="first_name">First Name *</label>
 </td>
 <td valign="top">
  <input  type="text" name="first_name" maxlength="50" size="30">
 </td>
</tr>
 
<tr>
 <td valign="top"">
  <label for="last_name">Last Name *</label>
 </td>
 <td valign="top">
  <input  type="text" name="last_name" maxlength="50" size="30">
 </td>
</tr>
<tr>
 <td valign="top">
  <label for="email">Email Address *</label>
 </td>
 <td valign="top">
  <input  type="text" name="email" maxlength="80" size="30">
 </td>
 
</tr>
<tr>
 <td valign="top">
  <label for="telephone">Telephone Number</label>
 </td>
 <td valign="top">
  <input  type="text" name="telephone" maxlength="30" size="30">
 </td>
</tr>
<tr>
 <td valign="top">
  <label for="comments">Comments *</label>
 </td>
 <td valign="top">
  <textarea  name="comments" maxlength="1000" cols="25" rows="6"></textarea>
 </td>
 
</tr>
<tr>
 <td colspan="2" style="text-align:center">
  <input type="submit" value="Submit">
 </td>
</tr>
</table>
</form>