电子邮件发送后如何进行php重定向?

So I am trying to make redirection after submitting the form to the home page. Fine, it works BUT there is one problem. The message is not displayed as user presses "Submit" button but is instantly redirected to the Home page. How can I make it so users first recieve "Thank you" message and than are redirected to the Home page?

<?php
$field_name = $_POST['cf_name'];
$field_email = $_POST['cf_email'];
$field_message = $_POST['cf_message'];

$mail_to = 'vladozabunov@gmail.com';
$subject = 'Message from a site visitor '.$field_name;

$body_message = 'From: '.$field_name."
";
$body_message .= 'E-mail: '.$field_email."
";
$body_message .= 'Message: '.$field_message;

$headers = 'From: '.$field_email."
";
$headers .= 'Reply-To: '.$field_email."
";

$mail_status = mail($mail_to, $subject, $body_message, $headers);

if ($mail_status) { ?>
  <script language="javascript" type="text/javascript">
    alert('Thank you for the message. We will contact you shortly.');
    window.location = 'contact_page.html';
  </script>
  <?php
}
else { ?>
    <script language="javascript" type="text/javascript">
      alert('Message failed. Please, send an email to gordon@template-help.com');
      window.location = 'contact_page.html';
    </script>
    <?php
}
header('Location: https://saprs.000webhostapp.com/index.html');
?>
$mail_to = 'vladozabunov@gmail.com';
$subject = 'Message from a site visitor '.$field_name;

$body_message = 'From: '.$field_name."
";
$body_message .= 'E-mail: '.$field_email."
";
$body_message .= 'Message: '.$field_message;

$headers = 'From: '.$field_email."
";
$headers .= 'Reply-To: '.$field_email."
";

$mail_status = mail($mail_to, $subject, $body_message, $headers);
if($mail_status == true){
    echo "<script>alert('Thank you for the message. We will contact you shortly.');</script>";
    header('Location:https://saprs.000webhostapp.com/contact_page.html');
}else{
    echo "<script>alert('Sorry! Please try again.');</script>";
    header('Location:https://saprs.000webhostapp.com/contact_page.html');
}

just remove the header from your code and replace with following

$mail_to = 'vladozabunov@gmail.com';
$subject = 'Message from a site visitor '.$field_name;

$body_message = 'From: '.$field_name."
";
$body_message .= 'E-mail: '.$field_email."
";
$body_message .= 'Message: '.$field_message;

$headers = 'From: '.$field_email."
";
$headers .= 'Reply-To: '.$field_email."
";

$mail_status = mail($mail_to, $subject, $body_message, $headers);

if ($mail_status) { ?>
 <script language="javascript" type="text/javascript">
  alert('Thank you for the message. We will contact you shortly.');
  window.location.href = 'index.html';
 </script>
 <?php
 }else { ?>
  <script language="javascript" type="text/javascript">
   alert('Message failed. Please, send an email to gordon@template-help.com');
   window.location.href = 'contact_page.html';
  </script>
 <?php } ?>

Replace

header('Location: https://saprs.000webhostapp.com/index.html');

with

echo "<script type='text/javascript'>window.location.href='https://saprs.000webhostapp.com/index.html'</script>";

Please do it with jQuery and ajax.

HTML and jQuery:

 <!DOCTYPE html>
    <html>
    <head>
        <title>Contact Form</title>
        <script src="//code.jquery.com/jquery-1.9.1.js"></script>
        <script type="text/javascript">
            function submitForm() {
             //Do validation and submit form
                $.ajax({
                  url: "mail.php",
                  type: "POST",               

                }).done(function( data ) {
                    alert(data);
                     if(data==1){
                         alert('Success');
                         window.location.href = 'test.php';//Your location 
                     }
                     else
                     {
                         alert('failed'); 
                     }
                });
                return false;
            }
        </script>
    </head>
    <body>
        <form method="post" id="mailData" name="mailData" onsubmit="return submitForm();">
            <label>Contact Form:</label><br>        
            <input type="submit" value="Submit" />
        </form>
        <div id="output"></div>
    </body>
</html>

Server-side PHP code:

<?php
$mail_to = 'vladozabunov@gmail.com';
$subject = 'Message from a site visitor '.$field_name;

$body_message = 'From: '.$field_name."
";
$body_message .= 'E-mail: '.$field_email."
";
$body_message .= 'Message: '.$field_message;

$headers = 'From: '.$field_email."
";
$headers .= 'Reply-To: '.$field_email."
";

$mail_status = mail($mail_to, $subject, $body_message, $headers);

if ($mail_status) { 
   echo 1;
}
else { 
   echo 0;  
}

?>