php mail

could you please check my script for security resons?
do you think I have an open space for the spammers to use my code to send spam emails from my web site?
I use also spry form validation with Dreamweaver.

<?php

$ipi = getenv("REMOTE_ADDR");

$httprefi = getenv ("HTTP_REFERER");

$httpagenti = getenv ("HTTP_USER_AGENT");

$httpref = $_POST['httpref'];

if(isset($_POST['email'])) {
     
    // EDIT THE 2 LINES BELOW AS REQUIRED
    $email_to = "my mail address";
    $email_subject = "contact form";
     
    function died($error) {
        // your error code can go here
        echo "there are some errors  ";
        echo "error<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'])) {
        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
         
    $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 />';
  }
  
  if(strlen($error_message) > 0) {
    died($error_message);
  }
    $email_message = "Web site contact form\n\n";
     
    function clean_string($string) {
      $bad = array("content-type","bcc:","to:","cc:","href");
      return str_replace($bad,"",$string);
    }
     
    $email_message .= "<br><br>" . "Ad-Soyad: ".clean_string($first_name)."\n"."<br>";
	$email_message .= "Email: ".clean_string($email_from)."\n"."<br>";
	$email_message .= "Cep Tel: ".clean_string($telephone)."\n"."<br>";
    $email_message .= "Sehir: ".clean_string($last_name)."\n"."<br>";
    $email_message .= "IP: ".clean_string($ipi)."\n"."<br>";
	$email_message .= "Refferal: ".clean_string($httprefi)."\n"."<br>";
	$email_message .= "Browser: ".clean_string($httpagenti)."\n"."<br>";
 
$headers .= 'Content-type: text/html; charset=utf-8' ."\n";
$headers .= "Reply-To: myotheremail@domain.com" ."\n";
$headers .= 'Cc: myotheremail@domain.com'  ."\n";
$headers .= 'Bcc: myotheremail@domain.com' ."\n";


// create email headers
//$headers .= "From: info@livcon.com.tr" . "\r\n";
//$headers .= "Reply-To: info@livcon.com.tr" . "\r\n"; 
//$headers .= "Organization: LIVCON\r\n"; 
//$headers .= "X-Priority: 3\r\n";
//$headers .= "X-MSMail-Priority: Normal" ."\r\n";
'X-Mailer: PHP/' . phpversion();
@mail($email_to, $email_subject, $email_message, $headers); 

echo "thank you";
}
?>

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