How to make it automatically find the browsing path-php Tutorial

How to make it automatically find the browsing path? Browsing path & nbsp; $ conf [& quot; Base_url & quot;] & nbsp; & quot; & nbsp ;. & nbsp; $ _ SERVER [& quot; SERVER_NAME & quot;]; this statement is not smart enough. how can I make it automatically find the browsing path in a subdirectory?
// Browsing path
$ Conf ["Base_url"] = "http: //". $ _ SERVER ["SERVER_NAME"];
This statement is not smart enough.
If I put it in a subdirectory, I want to change it:
// Browsing path
$ Conf ["Base_url"] = "http: //". $ _ SERVER ["SERVER_NAME"]. "/***";
Is there a better way?

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Print_r ($ _ SERVER)
Search by yourself
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Write directly

$ Conf ["Base_url"] = "/xxx ";
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What functions does the landlord want to implement ??