the value of image button problem

[universelittle]

I checked some documents and I found that when
<input type="image" NAME="buy" value="Buy">, the value is
ignored, only NAME is useful after you submitted the form, but
how to detect which image button was clicked at the server-side
if I want to know it is "buy" or "sell"?

Could anyone please tell me how to write that code? Thanks!

[sid]

either

//html form:
<input type="image" NAME="buy" value="Buy">
<input type="image" NAME="sell" value="Sell">

//variable handling
if (isset($buy)) {
    //...
}
elseif (isset($sell)) {
    //...
}

or

//html form:
<input type="image" NAME="buy" value="Buy">
<input type="image" NAME="buy" value="Sell">

//variable handling
switch ($buy) {
    case "Buy":
        //...
        break;
    case "Sell":
        //...
        break;
    default:
        break;
}

make sure to check the cases! (Buy != buy)