how to write to a file in php?

so I’m new to php and I’m working on a school task. Its a simple form that will take the inputs and write it to a txt file. But for me it doesn’t work. I tried to make a test version to see where the problem is and the error i get is 405 method not allowed. I have used fopen and fwrite too but they dont work too.

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Here is my html code

<!DOCTYPE html>
<html lang="No">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Index1</title>
</head>
<body>
 
  <form method="post" action="test21.php" name="myForm">
    klassekode <input type="text" id="klassekode" name="klassekode" required />
    <input type="submit" value="Fortsett" id="fortsett" name="fortsett" />
  </form>
 <a href="klasse.txt">Vis  klasser </a> 
               
</body> 
</html>

and my very simplified php code that i used to test test21.php

<?php
file_put_contents("klasse.txt", "test");
?>