Mine
Link: Easy to Modify Login Script with Hierarchical User Permissions and XML Account File
I created a flexible one a while ago that I would say is rather secure, if used correctly.

Alternative
Link: PHP Login Class
Daz also seems to have created one, but I don't know much about it. There are many comments for it, however.

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The man who comes back through the Door in the Wall will never be quite the same as the man who went out.