$_GET is not a function or language construct—it's just a variable (an array). Try:

<?php
echo $_GET['link'];

In particular, it's a superglobal: a built-in variable that's populated by PHP and is available in all scopes (you can use it from inside a function without the global keyword).

Since the variable might not exist, you could (and should) ensure your code does not trigger notices with:

<?php
if (isset($_GET['link'])) {
    echo $_GET['link'];
} else {
    // Fallback behaviour goes here
}

Alternatively, if you want to skip manual index checks and maybe add further validations you can use the filter extension:

<?php
echo filter_input(INPUT_GET, 'link', FILTER_SANITIZE_URL);

Last but not least, you can use the null coalescing operator (available since PHP/7.0) to handle missing parameters:

echo $_GET['link'] ?? 'Fallback value';

basename($_SERVER['REQUEST_URI']);

This will return all URLs with page name. (e.g.: index.php?id=1&name=rr&class=10).

You can use the parse_url() and parse_str() for that.

$parts = parse_url($url);
parse_str($parts['query'], $query);
echo $query['email'];

If you want to get the $url dynamically with PHP, take a look at this question:

Get the full URL in PHP