Diakritika odesílacího souboru formuláře

<?php
if(isset($_POST['email'])) {
     
    // CHANGE THE TWO LINES BELOW
    $email_to = "vodysm@seznam.cz";
     
    $email_subject = "CHYTRAPRACE zajemce";
         function died($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 .= 'Zadaná emailová adresa není správná.<br />';
  }
    $string_exp = "/^[A-Za-z .'-]+$/";
  if(!preg_match($string_exp,$first_name)) {
    $error_message .= 'Napište prosím své jméno.<br />';
  }
  if(!preg_match($string_exp,$last_name)) {
    $error_message .= 'Napište prosím své příjmení.<br />';
  }
  if(strlen($comments) < 2) {
    $error_message .= 'Musíte vyplnit položku Zpráva.<br />';
  }
  if(strlen($error_message) > 0) {
    died($error_message);
  }
    $email_message = "Detaily o zájemci:\n\n";
     
    function clean_string($string) {
      $bad = array("content-type","bcc:","to:","cc:","href");
      return str_replace($bad,"",$string);
    }
     
     
    $email_message .= "Jméno: ".clean_string($first_name)."\n";
    $email_message .= "Příjmení: ".clean_string($last_name)."\n";
    $email_message .= "Email: ".clean_string($email_from)."\n";
    $email_message .= "Zpráva: ".clean_string($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 -->
 
Děkujeme za Váš zájem! Na Vaši zprávu odpovíme do 2 pracovních dnů.
 
<?php
}
die();
?>