Well - I'll copy the "good" parts from the thread there:
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POSTED QUESTION:
In mozilla 1.0 and firefox, On clicking <input type="image" name="submit" value="1" src="whatever"> the following fields are posted to the server

submit.x=(x coordinate clicked on image)
submit.y=(y coordinate clicked on image)
submit=1

Now I can't seem to successfully test for the existence of submit.x in php (presumably cos of the "."
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RESPONSE:
I've done this many times. All I needed was this:

<INPUT name="submit" type=image src="images/subbutton.jpg" width="350" height="24" border="0">

if ($_POST['submit_x'])
{
……………

}

So add "_x" to whatever the name of the image button. In your case, it's also "submit_x
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RESPONSE 2:
When you submit your form with a graphic button, you will notice the .x and .y appended to the name of the button.

…snertal&crew%5B%5D=gosny&formbutton.x=37&formbutton.y=14

But if you were to submit the page and look at the results, however, you'd see:

formbutton_x = 37
formbutton_y = 14

Notice that the period (.) has been converted to an underscore (_). This may seem a little odd, but it's necessary because variable names in PHP can't have periods in them, so $formbutton.x would be an illegal variable name. In fact, any periods in form names — not just those for image buttons — are converted to underscores.