Okay. on the page that displays the div you can include the following PHP code IN PLACE OF THE DIV that is normally hidden. Let's say your variable is called "display" and has the value "yes" or "no." We will write a script that will find out if a value for "display" is being passed in the URL. If it is, and if the value is "yes," it will add 'style="display:visible"' to the <div> tag in question. If no variables are being passed, or the value is not "yes," it won't.
Let's say the following is the normal HTML display, and that the class "hiddendiv" has some styling we want to maintain when we change display to :visible (e.g. margins, padding, etc):
Using some PHP, we can do this instead!
it occurs to me now that I should have asked whether you were using the attribute "display" (display:none) or "visibility" (visibility:invisible) to hide your hidden div. Either wait, the code will still work, though you may have to change "display:block" to "visibility:visible."