How to prvent XSS attack in html output tag? [duplicate]

I’m new to html, and I’m trying to prvent XSS attack in html output tag like

.
for example here is a some simple html code to text XSS hacking

<!DOCTYPE html>
<html>
  <head>
    <link rel="icon" type="image/x-icon" href="icon.png">
  </head>
<body>
  <style>
    
    #edit {
      position: absolute;
      left: 50%;
      top: 90%;
      transform: translate(-50%, -50%);
      width: 250px;
      height: 50px;
      background-color: #34495E;
      font-size: 18px;
      color: white;
      
    }

    #texta {
      height: 90%;
      width: 100%;
    }
  </style>
  
  <?php
  $myfile = fopen("text", "r+") or die("Unable to open file!");
  if (filesize("text") == 0) {
      $con = null ;
  } else {
      $con = fread($myfile,filesize("text"));
  }
  
  ?>
  <p><?php echo$con; ?></p>

  <form action="save.php" method="post">
    <textarea id="texta" name="txt" rows="30" cols="150"><?php echo $con; ?></textarea>

    <input type="Submit" id= "edit" value="Save">
  </form>
  

</body>
</html>

and here is the save.php which save the text in textarea.

<?php

$text = $_POST["txt"];
$file = fopen("text", "w+");
fwrite($file, $text);
fclose($file);
header("location: index.php");
die();
?>

I hacked it with XSS so easly, what should I add to the code to prevent XSS hack?