php - 将变量传递给 mysql 查询结果为 "function query on null"

Question

我有一个类正在向其传递查询，但出现 fatal error

function query on null'

在尝试运行查询时发生。

我回显 $mysql_statement 并得到

select * from users 

这是预期的查询，但在查询语句的上下文中，它一直告诉返回 fatal error function query on null。

关于为什么合法存储在 $mysql_statement 中的“select * from users”被解释为 null 有什么想法吗？ (此处使用但未定义的属性在父类中定义):

page.php:

<?php
include("includes/database_classes.php");

new database_connection;
new database_query("SELECT * FROM users");

?>

...和database_classes.php:

<?php
/*----------------------------------------------------------------------------------------------------*/
    class database_connection
    {   
        protected $username = 'root';
        protected $password = '';
        protected $hostname = 'localhost';
        protected $database = 'assistant';  
        protected $database_handle;

        function connect()
        {
            try
            {
                $database_handle = new PDO("mysql:host=$hostname;dbname=$database", $username, $password);
            }

            catch (PDOException $e) 
            {
                print "Error!: " . $e ->getMessage() . "<br/>";
                die();
            }
        }
    }

/*----------------------------------------------------------------------------------------------------*/
class database_disconnection extends database_connection
{
    function disconnect()
    {
        $database_handle = null;
    }
}

/*----------------------------------------------------------------------------------------------------*/
    class database_query extends database_connection
    {
        protected $mysql_statement;

        function __construct( $mysql_statement )
        {
            $this->mysql_statement = $mysql_statement;

            echo $mysql_statement;

            foreach($this->database_handle->query("$mysql_statement")->fetch_assoc() as $row) {
                print_r($row);
            }
        }

    }

/*----------------------------------------------------------------------------------------------------*/

Answer 1

我认为您在尝试循环 SQL 查询结果时遇到问题，发生这种情况是因为您没有包含方法“fetch_assoc()”

而不是:

foreach($this->database_handle->query("$mysql_statement") as $row);

为什么不呢？

foreach($this->database_handle->query("$mysql_statement")->fetch_assoc() as $row)