错误：在非对象中调用成员函数prepare（）

I keep on getting this error

Call to a member function prepare() on a non-object in ..on line 134.

database.php

require_once("connect.php");

class Database {
    private $connect;

    function one() {
        $connectdb = $this->connect;
        $forms = new Forms();
        $forms->two();
    }
}

forms.php

require_once("connect.php");

class Forms {
    private $connect;

    function two() {
        $connectdb = $this->connect;

        $construct = $connectdb->prepare("SELECT child.*, parent.name as pname, parent.information as pinformation
            FROM strings child LEFT JOIN strings parent on child.pageid=parent.id WHERE child.sid=:id AND child.submittype='2'");
        $executequery = $construct->execute(array(':id'=>$id));
    }

}

The error happens on the line with the query. My guess is something to do with the private variable $connect. How do I solve this?

you need to instantiate first a new object of the Class Database and assign it to $connect:

$this->connect = new Database();

and then you might use it:

$connectdb = $this->connect;

Connect in your Forms class is not a database connector.

class Forms {
    private $connect;

    public function __construct(){
        $this->connect = Database::getConnection(); // we will define this method below
    }

    function two() {
        $connectdb = $this->connect;

        $construct = $connectdb->prepare("....query....");
        $executequery = $construct->execute(array(':id'=>$id));
    }

}

The database connector:

class Database {
    private static $connect;

    public static function getConnection(){
        if (self::$connect){
            return self:$connect;
        }
        self::$connect = new mysqli(...) or what ever connection u use

    }
}