Can you help me with a semi-advanced PHP form?

I know form basics, but say I wanted the contents of one drop down dependent on what the user selects in the drop down above it.

:Lets say we're talking about car makes and models.

I want the contents of the first drop down, "Make", to have Ford and Toyota as the 2 options.

Now, for the second one, "Model", I want the contents to be different depending on what the user selected. If they selected Ford, i want the second drop down to have selections like Taurus, Escort, and Explorer. Likewise, if they select Toyota, I want the second to have selections like Camry, Corolla, and Highlander.

What would be the easiest way to go about this? If it would take too long to explain, could you point me to a web page that explains it in detail? I have searched but i dont know any of the terms that go along with the settings i have described. Thanks, best answer to whoever helps me get the job done!