How to use PHP for form validation?

Upon submiting you will lose "?page=pizza_bestelling" of the link, you can set it in a hidden input with that value if you need it passed.

set the method to method="post" in the form

and on the same page you will need something like

function validate_form($data)
{
   //rules that will end with  return false; if they are not valid
   // example
   if (!isset($data["naam"]) || $data["naam"] == '') return false; // if the name is not set or it's empty

   // return true if all validation checks are passed
   return true;
}


if ($_SERVER["REQUEST_METHOD"] === "POST")
{
   $form_data['naam'] = $_POST['naam'];
   $form_data['woonplaats'] = $_POST['woonplaats'];
   // and so forth for each variable

   // run the validate_form function for the data you got through POST
   validate_form($form_data);

}else
{
 //form displayed here

}

You want to set up an errors array which will store any errors, and allow you to print them out if necessary.

Then you check each variable in the following manner:

$errors = array(); //initialise error array

if (empty($_POST['naam'])) {
        $errors[] = 'You forgot to add a name';
    } else {
        $name = trim($_POST['naam']);
    }

This checks if the naam input and writes a message to the errors array if it's empty, or assigns the value to the $name variable if not.

Once you've gone through all your inputs, you can then perform a check on the errors array:

if (empty($errors)) { // If everything's OK.
 //do what you need to do
} else {
echo '<p>The following errors occurred:<br/>';
        foreach ($errors as $msg) {
            echo "$msg<br/>\n";
        }
        echo '<p>Please try again</p>';
    }

This only checks for empty fields of course - you should implement more validation for valid emails etc. For useability purposes you should really look at a one-page solution with sticky fields though.

Do a for each as follows:

$allowed = array("naam","woonplaats","stratnaam","formnumbers","telefoonnummer");
foreach ($allowed as $key => $val){
  if($_POST[$key] == "")
    $errors = true;
}
if($errors){
  // don't submit
}
else {
  // submit 
}