Question

How to write a regular expression for link.php?id= that present in the URL

Answer 1

If your actually on that page i.e. link.php then you can use $_GET['id'] to get the ID param value.

If you need regex to match a string then you can use…

$str = "link.php?id=45";

if (preg_match("/link\.php\?id=(\d+)/", $str, $matches)) {
    echo $matches[1]; // would give you the number 45
}