PHP表单到多个收件人

I found and copied this code for sending an email via a form fill and it works, but I don't know anything about PHP coding.

I thought that I could add multiple addresses like ... $email_to: = "email1@email.com, email2@email.com, email3@email.com"; ... but this only sends to the last one in the list.

I know it seem simple to you guys, but I am not able to decipher the other posts about this subject, since the other code samples are not exactly like mine, and I know nothing about how the code works.

if(isset($_POST['email'])) {

    // EDIT THE 2 LINES BELOW AS REQUIRED 
    $email_to = "email1@email.com, email2@email.com, email3@email.com";
    $email_subject = "Someone registered on website.com";

    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['cell_phone']) ||
        !isset($_POST['text_me'])){
        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
    $cell_phone = $_POST['cell_phone']; // required
    $text_me = $_POST['text_me']; // 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($cell_phone) < 2) {
    $error_message .= 'Please enter your cell phone number.<br />';
  }
    if(strlen($text_me) < 2) {
    $error_message .= 'Please choose whether we can text you or not.<br />';
  }

  if(strlen($error_message) > 0) {
    died($error_message);
  }

    $email_message = "Form details below.

";


    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)."
";
    $email_message .= "Last Name: ".clean_string($last_name)."
";
    $email_message .= "Email: ".clean_string($email_from)."
";
    $email_message .= "Cell Phone: ".clean_string($cell_phone)."
";
    $email_message .= "Text Me?: ".clean_string($text_me)."
";

// create email headers
$headers = 'From: '.$email_from."
".
'Reply-To: '.$email_from."
" .
'X-Mailer: PHP/' . phpversion();
@mail($email_to, $email_subject, $email_message, $headers);  
?>```
... Please tell me what I can do to make it send to multiple emails. 
Thanks in advance!

Dan