They're pretty much the same, however in 3-Tier, the top level (presentation) never directly communicates with the bottom layer (data persistence).
In model-view-controller, theoretically the Model is supposed to 'notify' the View that it has changed so that the View can update. However, this is usually not an issue in most web applications because they are stateless. I'm not sure if any well-known PHP MVC architectures have Views that directly communicate with Models or vice versa, but if they don't it would be correct to say that they are in fact 3-Tier.
Wikipedia says:
At first glance, the three tiers may
seem similar to the MVC (Model View
Controller) concept; however,
topologically they are different. A
fundamental rule in a three-tier
architecture is the client tier never
communicates directly with the data
tier; in a three-tier model all
communication must pass through the
middleware tier. Conceptually the
three-tier architecture is linear.
However, the MVC architecture is
triangular: the View sends updates to
the Controller, the Controller updates
the Model, and the View gets updated
directly from the Model.
Source: Wikipedia: Multitier architecture
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