thanks status when mail send with PHP

I want to display a thanks message when mail is send with PHP

I tried something like

if(mail){echo "<script type='text/javascript'>alert('bedankt!');</script>";}

But I can't get the alert. I direct my form to mail.php but when the mail.php is done it redirects again to index.html with header('Location: ' . $_SERVER['HTTP_REFERER']);

Can this be the reason it doesn't show the alert? Or am I doing something wrong?

EDIT:

My code:

        <form action="mail.php" method="post" onsubmit="bedanktBericht()">
            <input class="invoeren" name="naamUser" type="text" placeholder="Naam" required>
            <input class="invoeren" name="nummerUser"  type="text" placeholder="Telefoonnummer" required>
            <input class="invoeren" name="onderwerpUser"  type="text" placeholder="Onderwerp" required>
            <textarea class="invoeren" type="text" name="opmerkingUser"  placeholder="Opmerking" required></textarea>           
            <button type="submit" id="contactButton" >Verstuur</button>
        </form>

<?php error_reporting(E_ALL); ini_set('display_errors', 1); 
ob_start(); 

$naamUser = $_POST['naamUser'];
$nummerUser = $_POST['nummerUser'];
$onderwerpUser = $_POST['onderwerpUser'];
$opmerkingUser = $_POST['opmerkingUser'];


$formcontent="Aanvraag formulier Domein naam \n
Naam: $naamUser \n
Telefoonnummer: $nummerUser \n
Onderwerp: $onderwerpUser \n
Opmerking: $opmerkingUser \n              
              ";              

$recipient = "email@email.nl";

$subject = "Terug Bellen";

$mailheader = "From:  \r\n";

mail($recipient, $subject, $formcontent, $mailheader) or die("Error!");


header('Location: ' . $_SERVER['HTTP_REFERER']); /* Stuur de browser naar www.site.nl */  

if(mail){echo "<script type='text/javascript'>alert('bedankt!');</script>";}
?>

UPDATE I want to execute the alert on the index.html page! Not mail.php! That's why I tried using the echo after the redirect.