PHP Contact script errors in one function - php

Question

I have HTML and PHP code for contact form. When someone type characters like this: š, đ, ž, č, and ć (UTF-8) characters, script reports few errors.
1. How can I add UTF-8 input in $first_name and $last_name input forms?
Next problem is, message of error or success is poping up by Java Script pop up. If I type in name box "šđžčć" and in last name box "šđžčć" script is reprting three mesages:
Wrong name
Wrong last name
Please try again (not exactly like this but some message)
And that means - three pop up windows! You get one - you close it, then second - close it too, etc etc, and it's frustrating. My secong question:
2. How can I rewrite this PHP and put all messages and errors in one function/value/message and make just one pop up says: Wrong name, last name, i love you etc.
PHP code:
<?php
if(isset($_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 "<script type='text/javascript'>alert('error')</script>";
echo "<script type='text/javascript'>alert('these errors apears below')</script>";
echo $error."<br /><br />";
echo "<script type='text/javascript'>alert('please fix errors')</script>";
die();
}
// validation expected data exists
if(!isset($_POST['first_name']) ||
!isset($_POST['last_name']) ||
!isset($_POST['email']) ||
!isset($_POST['telephone']) ||
!isset($_POST['comments'])) {
died(<script type='text/javascript'>alert('Submitted successfully!')</script>');
}
$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 .= '<script type='text/javascript'>alert('wrong email !')</script>';
}
$string_exp = "/^[A-Za-z .'-]+$/";
if(!preg_match($string_exp,$first_name)) {
$error_message .= '<script type='text/javascript'>alert('invalid firs name')</script>';
}
if(!preg_match($string_exp,$last_name)) {
$error_message .= '<script type='text/javascript'>alert('invalid last name')</script>';
}
if(strlen($comments) < 2) {
$error_message .= '<script type='text/javascript'>alert('invalid message')</script>';
}
if(strlen($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 -->
echo "<script type='text/javascript'>alert('Submitted successfully!')</script>"
<?php
}
?>