php中的“无法访问空属性”错误

Question

我很困惑我在下面的 php 代码中出错的地方。虽然，我在我的代码上看了很多次，但找不到为什么我会收到这个错误 'cannot access empty property' 。

class DBTest{
//declare variables
private $servername = "localhost";
private $username = "root";
private $password = "";
private $database = "avn_test";
private static $conn;
private $results;

//constructor
public function __construct(){
self::$conn = new mysqli($servername, $username, $password, $database);
if ($conn->connect_error) {
printf("Connect failed: %s\n", mysqli_connect_error());
exit();}
} //close constructor

public function executeQuery($query='') {
if(!empty($query)){
$query = self::$conn->real_escape_string($query);

此行出错：

$this->results = self::$conn->query($query) or die("数据库出错 连接".self::$conn->$error);

if( $this->results->num_rows > 0 ) {
$rowqry = array();
while($row = $this->results->fetch_object()) {
$rowqry[]= $row; } //close of while
$rarray['returnvar'] = $rowqry;
return $rarray;
} else {
return false; } // close of else
}//close of top if
else
return false;
} //close of function

function __destruct(){
self::$conn->close();}
} //close of class

//create an object of class DBTest
$test = new DBTest();
$q= "select * from test";
$tmp = $test->executeQuery($q);

if($tmp){
foreach($tmp as $key => $value){
echo $value;}
}
else
echo 'tmp var is empty';

Answer 1

在这一行：

$this->results = self::$conn->query($query) or die("Error in database connection".self::$conn->$error);

将self::$conn->$error 替换为self::$conn->error。

访问静态属性时需要$，但实例属性不需要。

Answer 2

$conn 函数中不需要$

self::$conn replace with self::conn
                           ^^^^^^