如何在PHP类中使用mysqli_query？

Question

How can i use $connect variable inside User class with out sending $connect as parameter or defines global $connect inside User class?

And I want to create only one connection is

$connect = mysqli_connect('localhost', 'root', '1234', 'mydatabase');

The error

Warning: mysqli_query() expects parameter 1 to be mysqli, null given in D:\\xampp\\htdocs\\mateocode\\index.php on line 9

Example code

<?php
    class User {    
        public $id;
        public $username;
        function __construct($user_id)
        {   
        $sql = "select id, username from user where id = '$user_id' limit 1";

        $result = mysqli_query($connect, $sql);    
        if(!$result) return false;

        if(mysqli_num_rows($result)>0){
            $data = mysqli_fetch_assoc($result);
            $this->id = $data['id'];
            $this->username = $data['username'];
        }else{ 
            return false; 
        }   
    }
}

$connect = mysqli_connect('localhost', 'root', '1234', 'mydatabase');

if($objUser = new User(1))
    echo $objUser->username;
else 
    echo 'There was an error or user not found!'; 

Answer 1

Try with -

<?php
    class User {    
        public $id;
        public $username;
        function __construct($user_id, $connect)
        {   
        $sql = "select id, username from user where id = '$user_id' limit 1";

        $result = mysqli_query($connect, $sql);    
        if(!$result) return false;

        if(mysqli_num_rows($result)>0){
            $data = mysqli_fetch_assoc($result);
            $this->id = $data['id'];
            $this->username = $data['username'];
        }else{ 
            return false; 
        }   
    }
}

$connect = mysqli_connect('localhost', 'root', '1234', 'mydatabase');

if($objUser = new User(1, $connect))
    echo $objUser->username;
else 
    echo 'There was an error or user not found!';