PHP validation (server side validation)

Hi, how can i add  PHP validation (server side validation) when form was successfully validated  and then to redirect to specific page? This si PHP from MDB: <?php $name = $_POST['name']; $email = $_POST['email']; $message = $_POST['message']; $subject = $_POST['subject']; header('Content-Type: application/json'); if ($name === ''){ print json_encode(array('message' => 'Name cannot be empty', 'code' => 0)); exit(); } if ($email === ''){ print json_encode(array('message' => 'Email cannot be empty', 'code' => 0)); exit(); } else { if (!filter_var($email, FILTER_VALIDATE_EMAIL)){ print json_encode(array('message' => 'Email format invalid.', 'code' => 0)); exit(); } } if ($subject === ''){ print json_encode(array('message' => 'Subject cannot be empty', 'code' => 0)); exit(); } if ($message === ''){ print json_encode(array('message' => 'Message cannot be empty', 'code' => 0)); exit(); } $content="From: $name nEmail: $email nMessage: $message"; $recipient = "youremail@here.com"; $mailheader = "From: $email rn"; mail($recipient, $subject, $content, $mailheader) or die("Error!"); print json_encode(array('message' => 'Email successfully sent!', 'code' => 1)); exit(); ?>

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