Better to have a single file of classes or multiple?

[elitis]

I'd like some opinions on this; and not specifically just for PHP. Is it better to have a single file composed of classes or is it better to have a single file for each class?

[Fou-Lu]

C# and Java require separate files. Multiple classes within are nested and owned by the outer class.
PHP should do the same, especially with chain-able autoload extensions and the use keyword with namespaces.