The concept you’re searching for is called “routing”, which is how a URL gets routed to particular content. This concept can be extremely complex (e.g. using a 3rd party routing framework like symfony), or as simple as checking:
$userId = $_GET['id'];
/* some mysqli/pdo/etc. code to grab info on that user */
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solved Not sure what this is exactly called but, page.php?id=5 [closed]