PHP - Returning MySQLi Object form __construct method

Warning: mysqli_query() expects parameter 1 to be mysqli

This is because $link is not actually an instance of mysqli; it is an instance of Connection.

You cannot return anything from the __construct(), as its purpose is only to build an instance of the Connection class itself, not to be a factory for other classes. The return $this->link does not do what you intended.

You have a couple of options.

You can make your class extend mysqli, wherein you are mostly just hard-coding the credentials and calling the parent::__construct() with them, and that eliminates the need for the __destruct() entirely:

// Merely extends mysqli, hard-coding your credentials
class Connection extends mysqli{
    // Your constructor has no params, and internally calls
    // the parent constructor
    public function __construct(){
        parent::__construct("localhost","root","","myDatabase");
    }    
}

The code you wrote to use this would then work immediately, but there isn't much added value, and I don't think this is really what you're going for.

Better option:

If you want to make a class which stores and manages a mysqli object, you can instantiate it but you cannot return it. You need to create a method to return it.

class Connection{
    private $link;

    // The constructor instantiates the mysqli
    public function __construct(){
        $this->link = new mysqli("localhost","root","","myDatabase");
    }

    // Other methods can operate on or manage the mysqli instance
    public function __destruct(){
        $this->link->close();
    }

    // Public accessor method to return the connection
    public function getConnection() {
        return $this->link;
    }
}

Then call the method to retrieve the connection.

$link = new Connection();
$sql = "SELECT * FROM atable";

// Call getConnection() to get the actual link
$query = mysqli_query($link->getConnection(), $sql);
$link->__destruct();

Return within __construct is useless because new operator returns always instance of given class.

To create an instance of a class, the new keyword must be used. An object will always be created unless the object has a constructor defined that throws an exception on error.

http://php.net/manual/en/language.oop5.basic.php#language.oop5.basic.new

Also, try to avoid of calling magic methods directly.