php - Redirect in same page and display message to user

one text

Solution:

As Ken Lee suggested in the comments, the best way of doing this is to use mail_handler.php itself as a form action. This will become your index.php:

<?php 
if(isset($_POST['submit'])){
    $to = "alex@gmail.com"; // this is your Email address
    $first_name = $_POST['fname'];
    $last_name = $_POST['lname'];
    $skola = $_POST['vysoka_skola'];
    $obor = $_POST['obor'];
    $prace = $_POST['prace'];
    $files = $_FILES['myfile']['name'];
    $kategorie = $_POST['kategorie'];
    //echo gettype($files);
    $all_thesis_string="";
    for($index=0;$index<count($_FILES['myfile']['name']);$index++){
        $all_thesis_string.=$_FILES['myfile']['name'][$index].","; //create a string with all bachelor attachments of user
    }

    $subject = "Form submission of ".$first_name." ".$last_name;
    $message = "User Details: \n\n Jméno: $first_name \n Příjmení: $last_name \n Vysoká škola: $skola \n Studijní obor: $obor \n Odkaz na bakalářskou práci: $prace \n Kategorie: $kategorie \n Bakalářská práce: $all_thesis_string";

    $headers = "From:" . $first_name;
    mail($to,$subject,$message,$headers);
    echo "Přihláška úspěšně odeslána. Děkuji " . $first_name . ".\n \n";
    include 'upload.php';
    // You can also use header('Location: thank_you.php'); to redirect to another page. 
    }
?>
<!-- The Form Section -->
<div style="background-color: #5A5377" id="form_section">
    <h2 class="w3-wide" align="Center"><font color="white"><strong><br>Přihláška</strong></font></h2>
    <form align="center" name="form1" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>" method="post" enctype="multipart/form-data">
      <ul class="errorMessages"></ul>
      <div class="row">
        <div class="col">
          <label for="fname"><font color="white"><strong>Jméno</strong></font></label></div>
        <div class="col">
          <input type="text" id="fname" name="fname" value="" required="required"><br></div>
      </div>
      <div class="row">
        <div class="col">
          <label for="lname"><font color="white"><strong><br>Příjmení</strong></font></label></div>
        <div class="col">
          <input type="text" id="lname" name="lname" value required="required"div>
      </div>
      <div class="row">
        <div class="col">
          <label for="vysoho"><font color="white"><strong><br>Vysoká škola</strong></font></label></div>
        <div class="col">
          <input type="text" id="vysoka_skola" name="vysoka_skola" value="" required="required"></div>
      </div>
      <div class="row">
        <div class="col">
          <label for="fname"><font color="white"><strong><br>Studijní obor</strong></font></label></div>
        <div class="col">
          <input type="text" id="obor" name="obor" value="" required="required"></div>
      </div>
      <div class="row">
        <div class="col">
          <label for="prace"><font color="white"><strong><br>Odkaz na bakalářskou práci</strong></font></label></div>
        <div class="col">
          <input type="text" id="prace" name="prace" value="" required="required"></div>
      </div>
      <div class="row">
        <div class="col">
          <label for="myfile"><font color="white"><strong><br>Bakalářská práce</strong></font></label></div>
        <div class="col">
          <input type="file" id="myfile" name="myfile[]" multiple="multiple" required="required"></div>
      </div>
      <div class="row">
        <div class="col">
          <label for="dropdown"><font color="white"><strong><br>Kategorie v soutěži</strong></font></label></div>
        <div class="col">
          <select id="kategorie" name="kategorie" required="required">
            <option value="science">Science</option>
            <option value="technology">Technology</option>
            <option value="science">Engineering</option>
            <option value="science">Mathematics</option>
          </select>
        </div>
        <br>
      </div>
      <div class="row">
        <div class="col"><input type="checkbox" id="gdpr" name="gdpr" value="" required="required">
          <label for="gdpr"><font color="white"><strong> I agree with GDPR</strong></font></label></div>
      </div>
      <div class="row">
        <div class="col">
          <input type="submit" name="submit" value="Poslat"><br><br></div>
      </div>
    </form>
  </div>
</div>
<!--END OF FORM-->

In the PHP script you can display whatever message you want to by just adding echo

Your second option is to target the form to an iframe:

<iframe name="myframe" id="frame1" src="thankyou.php" style="display:none"></iframe>
<form action="../thankyou.php" method="post" target="myframe">
<input type="submit" name="DoIt" value="DoIt">
</form>