[How do I secure this HTML Form and Email Script?]

Also, is there any server side scripting I can do to only let the script run if data is submitted from the form?  Thus preventing the script from running and sending a blank email if script is accessed?

[How do I secure this HTML Form and Email Script?]

Hi guys, sorry for such a newbish question.  Any help would be greatly appreciated.

 

HTML FORM:

<form action="form.php" method="post" onsubmit="return validateForm()" name="form">
<b>First Name:*</b>
<input type="text" name="first_name" size="50" />
<b>Last Name:*</b>
<input type="text" name="last_name" size="50" />
<b>Phone:*</b>
<input type="text" name="phone" size="50" />
<b>Email:*</b>
<input type="text" name="email" size="50" />
<p><b>What is your favorite color?*</b></p>
<p align="left">
<select name="se">
<option value="W">White</option>
<option value="G">Green</option>
<option value="Y">Yellow</option>
</select>
<input type="submit" value="Submit"/>
</form>

 

 

FORM.PHP script

<?php
$se = $_POST['se'];
$seURL = '';
switch ($se) {
  case 'W':
    $seURL = "http://url1.com";
    break;
  case 'G':
    $seURL = "http://url2.com";
    break;
  case 'O':
    $seURL = "http://url3.com";
    break;
  default:
    $seURL = "";
}
if ($seURL != "") {
/* Redirect browser */
/* make sure nothing is output to the page before this statement */
header("Location: " . $seURL);
}

// get posted data into local variables
$EmailFrom = "noreply@domain.com";                           
$EmailTo = "email@domain.com";                       
$Subject = "Form";                                         
$first_name = Trim(stripslashes($_POST['first_name'])); 
$last_name = Trim(stripslashes($_POST['last_name'])); 
$phone = Trim(stripslashes($_POST['phone'])); 
$email = Trim(stripslashes($_POST['email'])); 

// validation
$validationOK=true;
if (!$validationOK) {
  print "<meta http-equiv=\"refresh\" content=\"0;URL=error.htm\">";
  exit;
}

// prepare email body text
$Body = "";
$Body .= "first_name: ";
$Body .= $first_name;
$Body .= "\n";
$Body .= "last_name: ";
$Body .= $last_name;
$Body .= "\n";
$Body .= "phone: ";
$Body .= $phone;
$Body .= "\n";
$Body .= "email: ";
$Body .= $email;
$Body .= "\n";
$Body .= "color: ";
$Body .= $se;
$Body .= "\n";

// send email 
$success = mail($EmailTo, $Subject, $Body, "From: <$EmailFrom>");

// send email to user
if ($se=="W")
$EmailFrom = "noreply@domain.com";
$to = $email;
$subject = "form email";
$body = "thank you for filling out our form";
if (mail($to, $subject, $body, "From: <$EmailFrom>")) {
  echo("<p>Message successfully sent!</p>");
} else {
  echo("<p>Message delivery failed...</p>");
}

?>
[code]


MOD EDIT: [nobbc][code] . . . 

[/nobbc] tags added . . .[/code]