If a user fails form validation, then to make it easier for them we can do this:
if(isset($_POST['submit'])) {
$email = $_POST['email'];
//....
}
<input type="text" name="phone" value="<?php echo $phone; ?>" />
<input type="text" name="email" value="<?php echo $email; ?>" />
...
So in this case, if the user fails validation because they entered an email but not a phone number, then when the page refreshes upon submit and warns them of the missing phone, the email (which they did fill out) will already be in its place and won't require the user to retype that.
But how can I "remember" the values chosen by the user for select, radio and checkbox inputs?