PHP Error : Fatal error: Constant expression contains invalid operations

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());

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).

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.