Check if input field value exist in array, if so return to page with an error

Question

I have a form where user insert their names, once they submit the form, the page should check if the name provided already exists, if so, should return with an error, other wise

Answer 1

I got it from your comments that you have the following:

$_POST["user-submitted-name"]; // name from the form
$names = get_meta_values('user_submit_name'); // array with names not allowed

Then it looks like the easy way:

if (in_array($_POST["user-submitted-name"], $names)) {
    // posting not allowed
    echo '
Name already used';
} else {
    // posting is allowed
    // save posting here...
    // and then go to success page:
    header("Location: success.html"); // redirect
    exit(); // and stop here
}