Firstly, "Does not work" is very unspecific. Please explain what it's doing and what it's supposed to do and you're more likely to get an explaination on what your problem is.
Secondly, I don't think content put in a webpage with element.innerHTML should have tags. My suggestion would be to remove everything outside of the <?php ?> tag from display.php.
Thirdly, which files' contents did you paste? You mentioned display.php in your post, but your code refers to getCustomerData.php.
Fourthly, you shouldn't use the same file for AJAX requests and form actions. AJAX responses should be short and simple, while form actions usually include some headers and footers. You can include() the AJAX page from the form page, but don't put anything pointless (like tags) in the AJAX one.