PHP认为我的非空DSN == null

Question

I try to play with exceptions in PHP for my first time, but it does not go very well.

I have a variable named dsn, this contains the connection string of PDO / MySQL.

When I use var_dump on dsn it returns 'null', so i can assume that my dsn is empty.

But when I look in my code, the value of DSN is checked and when it equals to NULL it should throw an exception.

But, PHP does not throw an exception.

I already have checked for a typo, but dns does not exist in my code ;)

And due to this, I also can't connect to my database because I give PDO an empty DSN-string.

My code:

<?php
try {
    $host = 'localhost';
    $user   = 'root';
    $pass = '';
    $db = 'pridec';
    $charset = 'utf8';

    if ($host == NULL) {
        throw new Exception("Hostname is empty or is equal to null");
    }
    if ($user == NULL) {
        throw new Exception("Username is empty or is equal to null");
    }
    if ($db == NULL) {
        throw new Exception("Database is empty or is equal to null");
    }
    if ($charset == NULL) {
        throw new Exception("Charset is empty or is equal to null");
    }
    //Neither of these variables are working 
    //$dsn = "mysql:host=$host;dbname=$db;charset=$charset";
    //$dsn = "mysql:dbname=$db;host=$host;charset=$charset";
    $dsn = "mysql:dbname=$db";
    $opt = [
        PDO::ATTR_ERRMODE            => PDO::ERRMODE_EXCEPTION,
        PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
        PDO::ATTR_EMULATE_PREPARES   => false,
    ];


    if ($dsn = NULL) {
        throw new Exception("DSN can *never* be empty or equal to null / zero");
    }

    try {
        $pdo = new PDO($dsn, $user, $pass, $opt);
    } catch (PDOException $pdoe) {
        echo "<pre>";
        var_dump($dsn);
        var_dump($pdoe);
        echo "</pre>";
        throw new Exception("Cannot connect to the database because: ".$pdoe->getMessage());
    }

    require "classes/account.class.php";
    $account = new account($pdo);
} catch (Exception $e) {
    echo "An error occured:<br>";
        echo $e->getMessage();
} finally {
    ini_set("session.hash_function","sha512");
    session_start();
}
?>

I hope you guys can find the error to help me.

Answer 1

if ($dsn = NULL) {
    throw new Exception("DSN can *never* be empty or equal to null / zero");
}

should be

if ($dsn == NULL) {
    throw new Exception("DSN can *never* be empty or equal to null / zero");
}

what you have is "=" instead of "=="