The "Nodatabaseselected" error occurs when the class property is initialized by returning a reference to the function"

{Code...} found that this problem occurs if it is called in the function. The error output is: & quot; Nodatabaseselected & quot; and the program runs normally when I use it outside the class. Where is the error?

php
  connect_error) {        die("Connection failed: " . $conn->connect_error);    }    $conn->query("SET NAMES 'UTF8'");    return $conn;}?>

php
  conn=connectdb();        $result=$this->conn->query("SELECT * FROM 15_spring_new WHERE 1;");        var_dump($this->conn->error);    }}$bar = new Bar; ?>

This problem occurs when you call the function.
The error output is: "No database selected"
When I use it outside the class, the program runs normally.
Where is the error?

Reply content:

php
  connect_error) {        die("Connection failed: " . $conn->connect_error);    }    $conn->query("SET NAMES 'UTF8'");    return $conn;}?>

php
  conn=connectdb();        $result=$this->conn->query("SELECT * FROM 15_spring_new WHERE 1;");        var_dump($this->conn->error);    }}$bar = new Bar; ?>

This problem occurs when you call the function.
The error output is: "No database selected"
When I use it outside the class, the program runs normally.
Where is the error?