how to delete all cookies of my website in php

PHP setcookie()

Taken from that page, this will unset all of the cookies for your domain:

// unset cookies
if (isset($_SERVER['HTTP_COOKIE'])) {
    $cookies = explode(';', $_SERVER['HTTP_COOKIE']);
    foreach($cookies as $cookie) {
        $parts = explode('=', $cookie);
        $name = trim($parts[0]);
        setcookie($name, '', time()-1000);
        setcookie($name, '', time()-1000, "https://stackoverflow.com/");
    }
}

http://www.php.net/manual/en/function.setcookie.php#73484