prekates,

there is no way to block or control the back-button with php.
you may manipulate the entry of the last visited site (and only the
last visited!) in your browsers history using
javascript:location.replace(). open the site in a browser window
without a menubar and block the right mouse button. of course, modern
5- or more button mouses often doesn't need a right click to perform a
history.back() event.

giving a script full access to your browsers history opens an giant
security hole.

lorenz