不能在try catch塊之外使用屬性

Question

我正在嘗試在try catch塊中使用屬性。 try catch塊在類內部。 我想擴展一個特定的類，使其成為處理異常的類的子類。 問題是，當我嘗試使用子類中的那些變量時，它總是說未定義。 我必須刪除兩個類才能捕獲屬性。 通過在try catch塊內部之外添加return語句（我在其中添加return 1）來閱讀此處的其他答案后，它似乎不起作用，並且始終顯示未定義的變量。 有什么幫助嗎？

語言是PHP

沒有類的源代碼可以完美地工作：

    try
    {
        //$pdo variable to insert PDO object information
        $pdo = new PDO('mysql:host=localhost;dbname=studenti', 'root', '');

        //Set php to catch exceptions
        $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

        //Set UTF-8 for character encodings
        $pdo->exec('SET NAMES "utf8"');
    }
    //Catch error if unable to connect
    catch(PDOException $e)
    {
        //error variable
        $error = 'Unable to connect with database. ' . $e->getMessage();

        //include file once and show on screen error message
        include_once $_SERVER['DOCUMENT_ROOT'] . '/inc/error.inc.php';
        //Exit and don't process further
        exit();
    }

    //Another Exception handling
    try
    {
        //Select statement
        $sql = 'SELECT * FROM dega';
        $select = $pdo->query($sql);
    }
    catch(PDOException $e)
    {
        //error variable
        $error = 'Unable to select table. ' . $e->getMessage();

        //include file once and show on screen error message
        include_once $_SERVER['DOCUMENT_ROOT'] . '/inc/error.inc.php';

        //Exit and don't process further
        exit();
    }

帶類的源代碼不起作用：

<?php
    //PDO class, connection with MySQL database
    class Connect
    {
        function connection()
        {
        $pdo = null;
            try
            {
                //$pdo variable to insert PDO object information
                $pdo = new PDO('mysql:host=localhost;dbname=studenti', 'root', '');

                //Set php to catch exceptions
                $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

                //Set UTF-8 for character encodings
                $pdo->exec('SET NAMES "utf8"');
            }
            //Catch error if unable to connect
            catch(PDOException $e)
            {
                //error variable
                $error = 'Unable to connect with database. ' . $e->getMessage();

                //include file once and show on screen error message
                include_once $_SERVER['DOCUMENT_ROOT'] . '/inc/error.inc.php';
                //Exit and don't process further
                exit();
            }
        }
    }

    class Select extends Connect
    {
        function selection()
        {
            //Another Exception handling
            try
            {
                //Select statement
                $sql = 'SELECT * FROM dega';
                $select = $pdo->query($sql);
            }
            catch(PDOException $e)
            {
                //error variable
                $error = 'Unable to select table. ' . $e->getMessage();

                //include file once and show on screen error message
                include_once $_SERVER['DOCUMENT_ROOT'] . '/inc/error.inc.php';

                //Exit and don't process further
                exit();
            }
        }
    }

    //Output if successful
    $error = 'Database connection established.';
    include $_SERVER['DOCUMENT_ROOT'] . '/inc/error.inc.php';
?>

Answer 1

1. 您沒有子類。 對象繼承
2. 您沒有屬性。 性質

閱讀有關類和對象的信息 。

---

class Connect
    {
    protected $pdo = null;

    public function connection()
        {
        $pdo = null;
        try
            {
            $this->pdo = new PDO('mysql:host=localhost;dbname=studenti', 'root', '');
            $this->pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
            $this->pdo->exec('SET NAMES "utf8"');
            } catch (PDOException $e)
            {
            $error = 'Unable to connect with database. ' . $e->getMessage();
            include_once $_SERVER['DOCUMENT_ROOT'] . '/inc/error.inc.php';
            exit();
            }
        }
    }

class Select extends Connect
    {

    function selection()
        {
        try
            {
            $sql = 'SELECT * FROM dega';
            $select = $this->pdo->query($sql);
            } catch (PDOException $e)
            {
            $error = 'Unable to select table. ' . $e->getMessage();
            include_once $_SERVER['DOCUMENT_ROOT'] . '/inc/error.inc.php';
            exit();
            }
        }
    }

Answer 2

將$pdo放在connection()之前：

class Connect
{
    protected $pdo = null; // or var $pdo = null;
    function connection()
    {
     ...

在函數內部，它是一個局部變量，而不是類級別的屬性。

編輯 ：
PHP需要$this-><class-variable>來訪問函數中的類屬性。 （請在下面查看@Sectus的答案。）僅使用$ pdo即可在兩個方法中即時創建局部變量，但僅在selection()產生錯誤，因為這是在不初始化PDO()情況下調用query()的地方。 PDO()對象。