为什么在第二个函数调用PDO对象是NULL？ [重复]

Question

This question already has an answer here:

• Fatal error: Call to a member function prepare() on null 2 answers

I built the database connection with Object static method. Then each function that gets data from database will connect the database first. however, in the second function call "get_us_states()", var_dump($db) is "NULL", but in the first function call "get_para_from_table_bank()", var_dump($db) is "object(PDO)#1 (0) { }". Can sb know why?

The code is below.

class Database_connection {
        private static $dsn = 'mysql:host=localhost;dbname=xxx';

        private static $user = 'xxx';

        private static $pw = 'xxx';

        private static $option = array(PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION);

        private static $db;

        public function __construct() {}

        public static function getDB () {
            if (!isset(self::$db)) {
                try {
                    self::$db = new PDO(self::$dsn,
                                        self::$user,
                                        self::$pw,
                                        self::$option);
                } catch (PDOException $e) {
                    $error_message = $e->getMessage();
                    include('../errors/database_error.php');
                    exit();
                }
                return self::$db;
            }
        }
    }


    function get_para_from_table_bank(){
        $db = Database_connection::getDB();
        $query = 'SELECT * FROM form_content_bank WHERE current_form = 1 ORDER BY orders';

        try {
            $statement = $db->prepare($query);
            $statement->execute();
            $results = $statement->fetchAll();
            $statement->closeCursor();
            return $results;
        } catch (PODException $e) {
            $error_message = $e->getMessage();
            include('../errors/database_error.php');
            exit();
        } 
    }


    function get_us_states(){
        $db = Database_connection::getDB ();

        $query = "SELECT * FROM us_states ORDER BY id";

        try {
            $statement = $db->prepare($query);
            $statement->execute();
            $us_states = $statement->fetchAll();
            $statement->closeCursor();
            return $us_states;
        } catch (PODException $e) {
            $error_message = $e->getMessage();
            include('../errors/database_error.php');
            exit();
        }
    }


    function show_form(){
        //get form content from database
        $form_contents = get_para_from_table_bank();
           ***var_dump($db) in "get_para_from_table_bank()" is "object(PDO)#1 (0) { }"***
        $us_states = get_us_states();
           ***var_dump($db) in "get_us_states()" is "NULL"***

         ... (other code)
     }

     show_form();

After call show_form function, always show "Fatal error: Uncaught Error: Call to a member function prepare() on null ", and I found when call "get_us_states()", the $db is NULL.

Can sb give me some suggestions?

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Answer 1

Else case is missing from getDB function. when you call first time getDB there is no db instance. it goes in if bolck but in 2nd call its returning null

public static function getDB () {
        if (!isset(self::$db)) {
            try {
                self::$db = new PDO(self::$dsn,
                                    self::$user,
                                    self::$pw,
                                    self::$option);
            } catch (PDOException $e) {
                $error_message = $e->getMessage();
                include('../errors/database_error.php');
                exit();
            }
            return self::$db;
        }
        else return self::$db;
    }