Php 未捕获错误：调用成员函数prepare（）

致命错误：未捕获错误：在上调用成员函数prepare C:\xampp\htdocs\PDO\config\DB.php:21堆栈跟踪：0中为空 C:\xampp\htdocs\PDO\config\Student.php14:DB:：prepare'SELECT*FROM t、 ..'1c:\xampp\htdocs\PDO\index.php60:Student->readall 2 {main}在第21行的C:\xampp\htdocs\PDO\config\DB.php中抛出

DB.php

class DB{
    private static $pdo;

    public function connection(){
        if(isset(self::$pdo)) {

            try{
                self::$pdo = new PDO ("mysql:host=".DB_HOST.";dbname=",DB_USER,DB_PASS);
                echo "Conncetion successfully";

            }catch(PDOException $e){
                echo "Conection Failed............".$e->getMessage();
            }
        }
        return self::$pdo;
    }
    public static function prepare($sql){ // own prepare() method
        return self::connection()->prepare($sql); //pdo prepare()
    }
}

Student.php

include "DB.php";

class Student{
    //private $table = "tb_user";

    public function readall()
    {

        //$sql = "SELECT * FROM $this->table";

        $sql = "SELECT * FROM tb_user";
            $stmt = DB::prepare($sql);
            $stmt->exceute();
            return $stmt->fetchAll();
    }

}

---

将语句从

 if(isset(self::$pdo)) {

进入

如果pdo对象不存在，则需要创建它

！issetself:：$pdo否定它

 if( ! isset(self::$pdo)) {