why the validation did not work & jump to another page?

Question

I wrote a customer_display.php to validate data (only First Name so far), but no matter First Name field is empty or not, the webpage will jump to customer_search.php & did

Answer 1

You have no name attribute with the value of 'action', so your update never happens.

<form action="" method="POST">
    <input type="hidden" value="update_customer">
    <!-- rest of the form -->
</form>

Edited for clarity.