php错误在非对象上调用成员函数query(）

db_class.php

<?php
class db_mysql
{
    private $dbhost; 
    private $dbusername;
    private $dbpassword;
    private $db;
    //Class is called with all parameters to make a successful connection.
    //
    function __construct($dbhost,$dbusername,$dbpassword,$db)
        {
        global $dbh;
        try {
                $dbh = new PDO("mysql:host=$dbhost;dbname=$db", $dbusername, $dbpassword);
            foreach($dbh->query('show tables;') as $row) {
            print_r($row);
            }
            //$dbh = null;
        } catch (PDOException $e) {
            print "Error!: " . $e->getMessage() . "<br/>";
            die();
        }
        }
    //Function to execute a query in the database and return result in a array
    //
    function db_mysql_query($queryst)
    {   
        foreach($dbh->query($queryst) as $row) {
            print_r($row);
            }
    }
}

index.php:

<?php
include 'db_class.php';
$db_m = new db_mysql('localhost','root','','arsaas'); 
$db_m->db_mysql_query('show tables;'); 
?>

执行index.php会出现以下错误：注意：未定义的变量：第32行C:''examplep''htdocs''srry''db_class.php中的dbh

致命错误：在的第32行对C:''examplep''htdocs''srry''db_class.php中的非对象调用成员函数query（）

当dbh在类构造函数中被实例化并声明为全局变量时，为什么它说它是未定义的变量？

感谢您的帮助。提前谢谢。

AR