PHP：在非对象上调用成员函数GetUserCollection（）[关闭]

Apologies for posting this question - I know they have been asked and answered several times (i.e. https://stackoverflow.com/questions/1787561/call-to-a-member-function-on-a-non-object, etc), but non of the suggested solutions seem to work for me. This is what I have:

class Common {

    private $model;

    public function _construct() {
        $this->model = Model::GetInstance();
        //$this->model->GetUserCollection();
    }

    public function validateusername($username) {
        $userlist = $this->model->GetUserCollection();
        $result = true;

        if ($username == false) {
            $this->error = "Please enter username.";
            $result = false;
        }
    }

}

// This is my DB model

class Model {

    private static $instance = null;
    private $conn = false;

    private function __construct() {
        if (!$this->connect()) {
            print "Unable to connect to database!";
            exit;
        }
    }

    public function connect() {
        if ($this->conn == false) {
            $this->conn = mysql_connect(DBHOST, DBUSER, DBPASS);

            if ($this->conn) {
                if (!mysql_select_db(DBNAME, $this->conn)) {
                    print "DB Connection Error!";
                    exit;
                }
            }
        }
        return $this->conn;
    }

    public function GetUserCollection() {
        return UserCollection::GetInstance();
        // return new UserCollection();
    }

    public static function GetInstance() {
        if (!self::$instance) {
            self::$instance = new Model();
        }
        return self::$instance;
    }
}

Why is it exactly this line : $userlist = $this->model->GetUserCollection(); which is throwing me an error even though I am returning a new instance of it in the in the model Thanks in advance, JJ.

Use two underscores in your constructor name: __construct.