PDO和Class给出致命错误：调用成员函数查询（）[关闭]

Question

I am having some trouble to get database query using below code. I am first time using classes and even PDO for connection so no idea what is wrong. For you knowledge I am not so master but trying to learn class so I would appreciate if you can tell me if anything can improve for this too.

Error Message: Fatal error: Call to a member function query() on a non-object in C:\ pat \ to\ class.php on line 72

<?php

    // get database connection
    private static function _db_connect() {

        try {

            $db_con = new PDO('mysql:host='.$db_host.';dbname='.$db_name, $db_user, $db_pass);
            $db_con = null;

        } catch (PDOException $e) {

            print "Error!" . $e->getMessage(). "<br/>";

        }

    }


    // get database tables prefix
    public static function prefix() {

        $prefix = 'tb_'; // once done will be set dynamically

        return $prefix;
    }


    // get all users from db
    public static function get_users() {

        $db = self::_db_connect();
        $prf = self::prefix();

        $st = $db->query('SELECT * FROM '.$prf.'users'); // this is the line #72 where error

        while($row = $st->fetch(PDO::FETCH_ASSOC))
         $rs[] = $row;

        return count($rs) ? $rs : array();

    }    


?>

EDIT: I have remove null and return PDO object here

    // get database connection
    private static function _db_connect() {

        try {

            $db_con = new PDO('mysql:host='.$db_host.';dbname='.$db_name, $db_user, $db_pass);
            return $db_con;

        } catch (PDOException $e) {

            print "Error!" . $e->getMessage(). "<br/>";

        }

    }


    // get database tables prefix
    public static function prefix() {

        $prefix = 'tb_'; // once done will be set dynamically

        return $prefix;
    }


    // get all users from db
    public static function get_users() {

        $db = self::_db_connect();
        $prf = self::prefix();

        $st = $db->query('SELECT * FROM '.$prf.'users'); // this is the line #72 where error

        while($row = $st->fetch(PDO::FETCH_ASSOC))
         $rs[] = $row;

        return count($rs) ? $rs : array();

    }    


?>

Answer 1

You're not returning anything from your db_connect method, so it returns NULL by default. You're trying to call, in essence, NULL->query(), which obviously doesn't make sense.

Modify db_connect to return the PDO object it creates.