Check form

form.html

<form action="server.php" method="POST" id="contact_form">
    <input type="hidden" name="check_filelds" value="email,name,text" />
    <input type="hidden" name="check_form" value="" />
    <input type="text" name="email" value="" />
    <input type="text" name="name" value="" />
    <textarea name="text" rows="4" cols="20"></textarea>
    <input type="button" name="send" value="send" onclick="checkAndSendForm($('#contact_form'))"/>
</form>

script.js

function checkAndSendForm(objform) {
    $.post("/data.php", "act=check_form&" + $(objform).serialize(), function (res) {
        $("input[name=check_form]").val(res);
        $(objform).submit();
    });
}

server.php

function genCheckForm($data) {
    $ret = "";
    if (!empty($data['check_filelds'])) {
        foreach (explode(",", $data['check_filelds']) as $field) {
            $ret.=$data[$field];
        }
    } else {
        return md5(uniqid(time()));
    }
    return md5("sdgf4fhdfhw4sdeg" . $ret);
}

// Generate code
if ($_POST['act'] == "check_form") {
    echo genCheckForm($_POST);
    exit();
}

// Check form
if ($_POST['check_form'] != genCheckForm($_POST)) {
    echo "Invalid form!";
    exit();
} else {
    echo "Valid form!";
    exit();
}