Wait what? I can't follow you, how do those two code snippets differ? What's the reason to use the latter instead of the first one?

```php
public function __construct($dbName = 'foo')
{ 
    // .. 
}
```

```php
public function __construct($dbName = null)
{
    if (null === $dbName) {
        $dbName = 'foo';
    }

    // ..
}