错误：“mysqli_query（）期望参数1为mysqli，在......中给出null”“[duplicate]

Something doesn't work in my PHP code. The function mysqli_query() inside the function db_query() has no access to the previously defined variable link. Why?

<?php
$link = mysqli_connect($hostname, $username, $password,$database);

function db_query($sql) {
    return mysqli_query($link ,$sql);
}

db_query('SELECT name FROM users LIMIT 1');

My problem is that I always get this Error:

Warning: mysqli_query() expects parameter 1 to be mysqli, null given in ...

</div>

For people looking at this question in the future:

In PHP, you do not have access to variales outside of the function like you can see above.

Go to variable scope to learn more.

There are a lot of solutions for this problem, but I prefer one that I show you here (including PDO). If you create a class and initialize your connection when a new object of this instance is made, it will be very easy to query sql later.

<?php
class Users {
    private $_conn;
    public function __construct() {
        //initialize the connection
        $this->_conn = new PDO('mysql:host=localhost;dbname=DBNAME;charset=utf8', USERNAME, PASSWORD);
    }

    public function __destruct() {
        //close the connection when the database connection is not needed anymore
        $this->_conn = null;  
    }

    public function do_something() {
        $this->_conn->query("SELECT name FROM users LIMIT 1");
    }
}

$users = new Users();    //initialize the new object Users
$users->do_something();

This is a scope issue.

Must be function db_query($db_conn,$sql) {

$link = mysqli_connect("myhost","myuser","mypassw","mybd") or die("Error " . mysqli_error($link));
$query = "SELECT name FROM mytable");
$result = mysqli_query($link, $query);

I hope this works