Excuse me, why can't I run out here? Solution details must be adopted

If you are using a version of php7 or higher, this will not affect the warning. You should put the program in the xampp installation directory in a directory beginning with h, then go to 127.0.0.1/hello

• It tells you that the HelloWorld class is defined, not used;

// 你可以这样输出它
$helloworld = new HelloWorld();
echo $helloworld->index();

• In addition to its index method, the return value type is not declared; You need to follow the method with the return value declaration

public function index() : string {
        return "index234";
}

• You can refer to the following questions: https://ask.csdn.net/questions/7507130

This answer quotes ChatGPT

Please test this code

namespace app\controller;

class HelloWorld {
    public function index() {
        return 'index234';
    }
}