PHP-General - Contact form php

<form name="htmlform" method="post" action="ReportIssueForm.php">
<table width="450px">
</tr>
<tr>
  <td valign="top">
  <label for="first_name"><b>First Name *</b></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"><b>Last Name *</b></label>
 </td>
 <td valign="top">
  <input  type="text" name="last_name" maxlength="50" size="30">
 </td>
</tr>
<tr>
 <td valign="top">
  <label for="email"><b>Email *<font size='2'></b></label>
 </td>
 <td valign="top">
  <input  type="text" name="email" maxlength="80" size="30">
 </td>

</tr>
<tr>
 <td valign="top">
  <label for="comments"><b>Comments*</b></label>
 </td>
 <td valign="top">
  <textarea  name="comments" maxlength="1000" cols="25" rows="6"></textarea>
</td>

</tr>
<tr>
 <td colspan="4" style="text-align:center">
<br><br>
 <input type="submit" value="Submit">   
</td>
</tr>
</table>
</form>

<?php
if(isset($_POST['email'])) {

// CHANGE THE TWO LINES BELOW
$email_to = "myemail";     
$email_subject = "Reporting and issue";


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['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
$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";
$email_message .= "Comments: ".clean_string($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); 
?>

<!-- place your own success html below -->

Thank you for Reporting an issue. We will look into it and try to solve it as soon as possible.<br><br>


<?php
}
die(); 
?>