PSR1 recommends that each class should be in its own file to aid autoloaders.
Having each class in a dedicated file usually plays nice with PSR autoloaders
and is therefore a well established practice. If you use other autoloaders, you
might not want to follow this rule.
In camelCase names are written without any punctuation, the start of each new word
being marked by a capital letter. The whole name starts with a capital letter as well.
Thus the name database provider becomes DatabaseProvider.
Having each class in a dedicated file usually plays nice with PSR autoloaders and is therefore a well established practice. If you use other autoloaders, you might not want to follow this rule.