PHP Error : Fatal error: Constant expression contains invalid operations

Question

I am getting an error:

Fatal error: Constant expression contains invalid operations in config.php on line 214

That line was:

 protected static $dbname = 'mydb_'.$appdata['id'];

Whether I did any mistakes in quotes? Or somewhere else?

My search for the error message only showed a different source cause (a dynamic default value in a function definition).

Answer 1

From the official Php documentation :

Like any other PHP static variable, static properties may only be initialized using a literal or constant before PHP 5.6; expressions are not allowed. In PHP 5.6 and later, the same rules apply as const expressions: some limited expressions are possible, provided they can be evaluated at compile time.

So you cannot initialize a static variable with another variable. Replace $appdata['id'] with a constant string or remove the static attribute.

This is because all static declarations are resolved in compile-time, when the content of other variables is not known (see this other page of official doc).

Answer 2

Unless you mess with reflection, the only way I can think of to have a static private/protected class property with a dynamically generated value is to calculate it outside the class:

class Foo
{
    protected static string $dbname = DBNAME;

    public static function debug(): string
    {
        return Foo::$dbname;
    }
}

$appdata = [
    'id' => 31416,
];
define('DBNAME', 'mydb_' . $appdata['id']);
var_dump(Foo::debug());

In your precise use case, however, it's possible that there's simply no good reason for the property to be static. In that case, it's as straightforward as using the constructor:

class Foo
{
    protected string $dbname;

    public function __construct(array $appdata)
    {
        $this->dbname = 'mydb_' . $appdata['id'];
    }

    public function debug(): string
    {
        return $this->dbname;
    }
}

$appdata = [
    'id' => 31416,
];
$foo = new Foo($appdata);
var_dump($foo->debug());

Answer 3

This is because a static variable contains a constant value in it. But in your case:

protected static $dbname = 'mydb_'.$appdata['id'];

$appdata['id'] is dynamic that can change its value during the execution. That's why the error is showing.

Answer 4

I had this error and my fix was to not declare a date within a class property array

public static $config_array = array(
    'start_date' => date('Y-m-d H:i:s') // No can do
);

Answer 5

In my case the solution to this issue was to create a singleton class so the variable's value is initialized (calculated) only once and still have access to it whenever I need.

(Extra info: The singleton is a design pattern which stricts you to have only 0 or 1 instance of your class in your entire program. There are several design pattern, I collected 115 of them so far on a single image)

In your case the code would look like this:

class DataBaseConfig {
                 
    private static $instance;
    private $dbname ;
                
    private final function __construct() {
        $this->dbname = 'mydb_'.$appdata['id'];
    }
    
    public static function getName() {
        if (!isset(self::$instance)) {
            self::$instance = new DataBaseConfig();
        }
        return self::$instance->dbname;
    }
}

You can use it like DataBaseConfig::getName() for the shortest / simplest version.

In reality you have more complex classes and functions so by defining a getInstance() function you will avoid repeating the <check instance> - <create instance process> lines for every function you add.

(Extra info: 2 synonym paradigms DRY - Don't Repeat Yourself and DIE - Duplication Is Evil)

This is the refactoring you need:

public static function getInstance() {
    if (!isset(self::$instance)) {
        self::$instance = new PhpStarter();
    }
    return self::$instance;
}

public function getName() {
    return $this->dbname;
}

Use it like $db = DataBaseConfig::getInstance(); then $db->getName() and $db->getSomethingElse() for the other functions.

Answer 6

For your information:- I got the same error by using some characters in a constant expressions.

public static $dbPassword="mAkE-34@-||sR";

This is what caused the error and I removed this || which is the logical OR operator characters from the string and it worked.