Use of PHP Form Validation

<?php
  // define variables and set to empty values
  $nickname = $anon = $feedback = '';

  if ($_SERVER['REQUEST_METHOD'] == 'POST') {
    $nickname = test_input($_POST['nickname']);
    $feedback = test_input($_POST['feedback']);
    $anon = test_input($_POST['anon']);
  }

  function proc_input($input) {
    $input = trim($input);
    $input = stripslashes($input);
    $input = htmlspecialchars($input);
    return $input;
  }
?>