I created contact form it is working OK but there is one problem after submitting the form the fields become not clean. all the info stays there?

Question

I have build my own contact form for first time just to learn. this is my php code :

<?php

// define variables and set to empty values
$nameErr = $emailErr = $phoneErr = $contentErr = "";
$name_sender = $email_sender= $phone = $content =$succes= "";

  if ($_SERVER["REQUEST_METHOD"] == "POST") {

    
  

    // post
    $name_sender = strip_tags($_POST['name_sender']);
    $email_sender = strip_tags($_POST['email_sender']);
    $phone = strip_tags($_POST['phone']);
    $content = strip_tags($_POST['content']);
        
    // validate name
     if (empty($_POST["name_sender"])) {
    $nameErr = "Name is required";
  } else {
    $name_sender = test_input($_POST["name_sender"]);
    // check if name only contains letters and whitespace
    if (!preg_match("/^[a-zA-Z-' ]*$/",$name_sender)) {
      $nameErr = "Only letters and white space allowed";
    }
     }
    
    
 
    
    
    if (empty($_POST["email_sender"])) {
    $emailErr = "Email is required";
  } else {
    $email_sender = test_input($_POST["email_sender"]);
    // check if e-mail address is well-formed
    if (!filter_var($email_sender, FILTER_VALIDATE_EMAIL)) {
      $emailErr = "Invalid email format";
    }
  }

    
    

    // validate phone
    if (empty($_POST["phone"])) {
        $phoneErr = "Phone is required";
    } else {
         $phone = test_input($_POST["phone"]);
        // check if phone is well-formed
        if(!preg_match("/^[0-9\s\-]+$/", $phone)) {
            $phoneErr = "Invalid phone number";
        }
    }


    // validate content
    if (empty($_POST["content"] )) {
        $contentErr = "Content is required";
    } else {
    $content = test_input($_POST["content"]);
  }

    // send
        if ( empty($nameErr) && empty($emailErr) && empty($phoneErr) && empty($contentErr) ) {
        // Variables
        $to = 'johannes@webdesignleren.com';
        $subject = 'Contact form';
        $success = 'Thank you! You will receive a response as soon as possible.';

        // Message content      
         $content .= '';    

        // Headers
        $headers     = 'From: ' .$name_sender. ' <' .$email_sender. '>' . PHP_EOL;
        $headers    .= 'Reply-To: <' .$email_sender. '>' . PHP_EOL;
        $headers    .= 'Content-Transfer-Encoding: 8bit' . PHP_EOL;
        $headers    .= 'Content-Type: text/plain; charset=UTF-8' . PHP_EOL;

        // Send email
        mail($to, $subject,$content, $headers); 
        echo $success;
    
}
            
      
  }




function test_input($data) {
  $data = trim($data);
  $data = stripslashes($data);
  $data = htmlspecialchars($data);
  return $data;
}
    


// form


?>

and this is the HTML code:

<div class="container">
<form id="contact" method="post">  

<h3>Quick Contact</h3>
<h4>Contact us today, and get reply with in 24 hours!</h4>
<fieldset>
<input placeholder="Your name" type="text" name="name_sender" value="<?php echo $name_sender;?>">

<span class="error">* <?php echo $nameErr; ?></span>
</fieldset>
<fieldset>
<input placeholder="Your Email Address" type="text" name="email_sender" value="<?php echo $email_sender;?>">
 
<span class="error">* <?php echo $emailErr; ?></span>
</fieldset>
<fieldset>
<input placeholder="Your phone number" type="text" name="phone" value="<?php echo $phone;?>">

<span class="error">* <?php echo $phoneErr; ?></span>
</fieldset>
<fieldset>
<textarea placeholder="Type your message here.." type="text" name="content"><?php echo $content;?></textarea>
<span class="error">* <?php echo $contentErr; ?></span>
</fieldset>
<fieldset>
<button name="submit" type="submit" value="reset" id="contact-submit">Submit</button></fieldset>

    
</form></div>

I can send and recieve an email everything is OK . but there is one problem after submitting and sending data all the data remains in the fields. the fields doesn't clean.

I see I can use reset button but I don't want that .I want that when customer sends an email all the fields become clean . I think this is the way of professional contact form. I searched in Google but I couldn't find something that satisfies me. I know I can do it with JS with ad listener event or sothing like this . but I want to achieve with php if it is possible . can you help me to achieve my goal exactly?

2- I see also after I insert many names or emails or phone numbers they all stay there as auto fill in the fields I want also to clean those informations after submitting the form!