I am new to PHP. In the path of learning PHP language, I notice that, some website would this kind of URL:
What is the "?" symbol used for?
If I were develop a php website, must I use it in my URL? For example, after a user(roa3) successful logged in, I will redirect to "www.website.com/profile.php?user=roa3" instead of "www.website.com/profile.php"
What are the advantages and disadvantages of using it?
Good questions, briefly,
"?" stands for the start of querying string which contains the data to be passed to the server. in this case you are passing user=roa3 to profile.php page. You can get the data by using $_GET['user'] within profile.php. querystring is one of the methods to send data to the server from client agent. The other one places the data in HTTP body and POST to the server, you don't see the HTTP POST?data directly from browser.
querystring can be edited by user and it is visible to the public. If www.website.com/profile.php?user=roa3 is intended to be public then it is fine, otherwise you may want to use session to get current user's context.
it is a flexible way to pass data to the server, but it is visible and editable to the users, for some sensitive data, at least produce some kind of hash before attaching it to the querystring, this prevents users to edit it or understanding the meaning of it. However this doesn't prevent a decent hacker to do something wrong about your website. Different browsers support different max length of URL, the lengthy URL is made up by those querystring parameters. If you want to send large amount of data, place the data in the HTTP body and POST to the server.