Checking for unfilled fields with PHP - php

Question

I am creating a checkout page that requires the client to fill out his personal information as well as his credit card details (this part using stripe).
I was wondering, what is the best way to check whether the fields are filled up or not? Shall I do it in the processingPayment.php that $_POSTs the fields and processes payment, and in case the fields were not filled, I would redirect back to checkout?
Or is it a better idea to use js to check on the spot before submitting the form?
if in the processing page, I would try something like this:
if (empty($firsName) || empty($lastName) || empty($address) || empty ($city) || empty ($state) || empty($zip))
{
header('Location: checkout.php');
}
But I would need to re-send the values that were entered so the checkout page receives them and the user doesn't have to re-fill every field again...

Answer 1

Something like this?
foreach($_POST as $key=>$val) {
if( empty($val) ) {
echo "$key is empty";
}
}
The best method with PHP is to have an array of possible arguments:
$array = array('firstName', 'lastName');
foreach($array as $val) {
if( empty($_POST[$val]) ) {
echo "$val is empty";
}
}
Otherwise, client side validation works too, but can always be disabled. To be completely safe, use both client and server side.

Answer 2

You can use the session to store the entered data, but you would need to check each value separately:
PHP
<?php
session_start();
foreach ($_POST as $key => $value) {
if (strlen(trim($value)) <= 0) { //You could replace '0'
$_SESSION[$key] = $value;
}
}
?>
FORM
<form>
First name: <input type="text" value="<?php $_SESSION['firstName'] ? $_SESSION['firstName'] : ''; ?>" placeholder="First Name" />
....
</form>
The $_SESSION['firstName'] ? $_SESSION['firstName'] : ''; is the same as
if ($_SESSION['firstName']) return $_SESSION['firstName']
else return '';
it is more readable in the HTML(View) that the full if statement

Answer 3

$var = isset($_POST['field']) ? $_POST['field'] : '';
$var2 = isset($_POST['field2']) ? $_POST['field2'] : '';
// and so on
if( empty($var) || empty($var2) )
{
//it's empty
}