filename for class files: file.class.php or file.php?

There is no standard per se for naming class files, however file.class.php sends the message that it contains class files, opposed to file.php which may give the impression of a regular file.

My suggestion is to use one convention and be consistent with it.

Update:

A good point to note, as AJ, had stated, the *.php file extension is preferred over *.inc file extensions. Reason being that *.inc files are displayed on the client's browser as plain text and if you were to store any configuration files or private information (especially in relation to your database, yikes) it could potentially be accessed by anyone.

Actually this method of naming files came from developers who combines both procedural & OOPs in a same built. To differentiate procedural files and class files they did this. There is no standard convention. But I will suggest you to use filename.php and name the class as same as file name, just as Java convention. Keep only one class definition in a file.

@Derk: It can also be like this, there is no benefit of adding class. in file name ;)

 function __autoload ($class) {

      require_once $class . '.php';

  }

  $pizza = new pizza ();