Passing price variable to PayPal with custom button

Question

I have a form and a custom PayPal button, but how do I pass the value/price variable to PayPal?

<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
    <input type="hidden" name="cmd" value="_s-xclick">
    <input type="hidden" name="hosted_button_id" value="ZEFZFYBY2SZB8">
    <input type="image" src="https://www.paypal.com/en_US/i/btn/btn_paynowCC_LG.gif" border="0" name="submit" alt="PayPal - The safer, easier way to pay online!">
    <img alt="" border="0" src="https://www.paypal.com/en_US/i/scr/pixel.gif" width="1" height="1">
</form>

I have a variable $total = "238.00";

Answer 1

The previous code did not work for me. After much headache I finally figured out you have to go into PayPal and on step 2 of creating the button code make sure you click the unhosted button, and then copy the unencrypted button code, which will give you something like this (I blanked out my business value for security):

<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
    <input type="hidden" name="cmd" value="_xclick">
    <input type="hidden" name="business" value="XXX">
    <input type="hidden" name="lc" value="CA">
    <input type="hidden" name="item_name" value="Tangled Roots">
    <input type="hidden" name="button_subtype" value="services">
    <input type="hidden" name="no_note" value="0">
    <input type="hidden" name="cn" value="Add special instructions to the seller">
    <input type="hidden" name="no_shipping" value="2">
    <input name="amount" value="16.99">
    <input type="hidden" name="currency_code" value="CAD">
    <input type="hidden" name="bn" value="PP-BuyNowBF:btn_buynowCC_LG.gif:NonHosted">
    <input type="image" src="https://www.paypalobjects.com/en_US/i/btn/btn_buynowCC_LG.gif" border="0" name="submit" alt="PayPal - The safer, easier way to pay online!">
    <img alt="" border="0" src="https://www.paypalobjects.com/en_US/i/scr/pixel.gif" width="1" height="1">
</form>

This code creates a button where the user can input the amount, which by default starts at 16.99, but you could just as easily replace that with a PHP variable.