Source of: /ADI/more_sessions-withCookies/input_validation.php

<?php
session_start();
$_SESSION['firstname'] = $_POST['firstname'];
$_SESSION['lastname'] = $_POST['lastname'];
$_SESSION['phone'] = $_POST['phone'];
$_SESSION['try']++;

//set-up an empty array to hold errors
$errors=array();

//validate data
if (empty($_POST['firstname']))
  $errors['firstname'] = 'the first name field can not be blank';

if (empty($_POST['lastname']))
  $errors['lastname'] = 'the last name field can not be blank';

if (!ereg('^([0-9]){9}$',$_POST['phone']))
   $errors['phone'] = 'the phone number must have 9 digits';

//store the errors as a session variable
$_SESSION['errors']  = $errors;

if(!empty($errors)) {
  // there are errors: try again
  header('Location: entry_form.php');
  exit;
}
else {
 //there are no errors: life goes on
 //destroy session
   session_destroy();
 
   printf("<html>\n");
   printf("<head>\n");
   printf("<title>Success</title>\n");
   printf("</head>\n");
   printf("<body>\n");
   printf("<p>Your data is valid and is ready for further processing</p>\n");
   printf("<p>A sessao terminou. Clique aqui para <a href=\"entry_form.php\">recomecar de novo</a>.</p>\n");
   printf("</body>\n");
   printf("</html>\n");

}

?>