You will need to get a PayPal button from the paypal website. From what I remember, there are two types or button, an encrypted one or an unencrypted one. I think you need an unencrypted one if you wish to plug in your own php variables into the paypal form.
It uses hidden fields to pass paypal the payee and product details. E.g:
<form
action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_xclick" />
<input type="hidden" name="business" value="paypal@mywebsite.com" />
<input type="hidden" name="return"
value="http://www.mywebsite.com/thankyou.html" />
<input type="hidden" name="item_name" value="Training Documents" />
<input type="hidden" name="amount" value="25.00" />
<input type="submit" value="Buy Now!" />
</form>
Here's a good link for some requisite research http://en.csharp-online.net/Encrypted_Website_Payments%E2%80%94PHP_Developers In addition, you may want to have a look at Instant Payment Notificaton (IPN) https://www.paypal.com/ipn