function defined with variable

I have the following code to define a constant with the value of a variable:

define('DB_NAME', $row['co_database_name']);

and use it in a connection string

$dsn = "mysql:host=" . DB_HOST . ";dbname=" . DB_NAME . ";charset=" . DB_CHARSET . ";port=" . DB_PORT;
$opt = [
   PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
   PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
   PDO::ATTR_EMULATE_PREPARES => false,
   PDO::ATTR_PERSISTENT => true,
   ];
$conn = new PDO($dsn, DB_USER, DB_PASS, $opt);

but gives error Undefined variable: conn

If I do it directly with the name of the database ... like this:

define('DB_NAME', 'nombre_base_datos');

It works well, why? Is there another way to do it?

Thank you.