hidden post parameter , php curl

I'm trying to replicate the browser po开发者_如何学编程st parameters on http://www.ebayclassifieds.com/m/PostAd?scrid=3465450-2253858851033189948 but for some reasons I can't find where the values for 2 of them are comming from . The parameters are btn-previwe-ad.x and btn-previwe-ad.y but I can't find such as parameters in the html source itself or any hidden value.

They represent the x and y position of the mouse pointer relative to the element when clicked, and are added automtically when submitting a form through an <input type="image"> click:

<input name="btn-previwe-ad" ... alt="Preview your ad" type="image">

From the HTML 4.02 specification (Source):

When a pointing device is used to click on the image, the form is submitted and the click coordinates passed to the server. The x value is measured in pixels from the left of the image, and the y value in pixels from the top of the image. The submitted data includes name.x=x-value and name.y=y-value where "name" is the value of the name attribute, and x-value and y-value are the x and y coordinate values, respectively.

They are the pos x and y of the mouse when you clicked, relative to the clicked button. They are added by default without the need of extra code.