How can I use client and server-side validation while minimizing duplication?

Create a function i PHP that validates the fields and set it up in a generic way. So you can use it both on the server side as on the client side.

For the clientside use jquery to make some AJAX calls to the PHP validation function.

Definitely do not use client side datetime checking for any purposes. You are right on that one.

So you want to write one code for each validation.

So go ahead and code a function in php which does email validation, let's say: validate_email().

Once the user presses submit, you can send all the data to the php code using ajax and have it validated.

Once the user passes the submit phase now you can use the same function validate_email() to validate the $_POST.

This way you write one code, and it is easier to maintain.

Is this slower compared to javascript? Hard to say. Remember, putting an additional jquery validation plugin has its load on the page too. Here is a popular validation plugin for jquery, it is 25KB in its minified version. Now your code is tidy and easier to maintain. If this is a homepage, and assuming you would opt in to use a plug-in, you just saved 25KB load on your probably most valuable landing page.

If your validation rules can be described in terms of regular expressions, you can define them in one place and use the same regex in your php code that you use in your generated javascript.

Some pseudo-code as a possible approach:

In PHP:

function validate() {
    // Use $_POST to validate whatever items in whatever fashion you want
    // and echo a message or some JSON array etc...
}

In javascript (assuming jQuery):

$('#formButton').click(function(event){
    event.preventDefault();
    $.post('/url/of/page', $(this).serializeArray(), function(response){
        // Assuming JSON data, but could just be a string or message,
        // check for response.success or whatever from validation
        if(response.success) $('#form').submit();
        else // Some error message or action
    });
});

This, while perhaps some overhead in AJAX calls, allows you to use any validation method you would want to and keeping the code the exact same whether from an actual POST or via AJAX. A simple response from a $_POST iteration from PHP, though, is not that much overhead. This also allows you to run queries if you need to - of course you'd want to keep them simple since everything would run twice.

at first i can say never trust user... user can change java script... so server side validation is require

second: why we use client side validation? for speed.

i think you should use both way . cuase you dont want waste user time

validate with js and sent data. in admin if there is wrong data exit user.