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).