how to make live preview form in PHP with javascript or ajax?

hi
i saw some sites where when you type in a text box of a form you can see it live in the side of the box and only show if the text-box has atleast one character.

but when i tried to find the source code got nothing, can anyone help me with this please to use in PHP page, thx in advance.

you can view the image of sample at: [url]http://0point5.com/live-form.JPG[/url]

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