Fatal error: Call to a member function query() on null in E:

<?php

class DatabaseConnection {

    public $dbconnection;
    public $dbhost;
    public $dbuser;
    public $dbpass;
    public $dbname;

// Create connection
    public function __construct($dbhost, $dbuser, $dbpass, $dbname) {
    $this->dbconnection = new mysqli($this->dbhost = $dbhost, $this->dbuser = $dbuser, $this->dbpass = $dbpass, $this->dbname = $dbname);
    }

}

class sql extends DatabaseConnection {

    public $sql;

    public function __construct($sql) {
        if ($this->dbconnection->query($this->sql=$sql) === TRUE) {
        echo "New record created successfully";
        } else {
        echo "Error: " . $this->sql . "<br>" . $this->dbconnection->error;
        }
    }

}



?>