PayPal buy now button with IPN - php

Question

I am new to PHP , and PayPal.
I need to do crowdfunding site.
Here I have a form where user can choose pay method :
Pay with terminal
Bank transfer
Paypal
Input with amount that user want to donate for project, and some fields like name, surname , phone etc.
Now action of my form is example.com/pay.php, here I register pay in my database with status "in progress" and i want after it to redirect user to PayPal page.
How to redirect user to PayPal with necessary data (see in PayPal button code) , after I add the transaction to my database?
Code of of my PayPal button:
<form action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post" target="_top">
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="BJKELCPG5WDSN">
<input type="hidden" name="lc" value="MD">
<input type="hidden" name="item_name" value="Help African child">
<input type="hidden" name="item_number" value="2299">
<input type="hidden" name="amount" value="67.00">
<input type="hidden" name="currency_code" value="EUR">
<input type="hidden" name="button_subtype" value="services">
<input type="hidden" name="no_note" value="1">
<input type="hidden" name="no_shipping" value="1">
<input type="hidden" name="rm" value="1">
<input type="hidden" name="return" value="http://example.com/test/thanks.php">
<input type="hidden" name="cancel_return" value="http://example.com/test/">
<input type="hidden" name="bn" value="PP-BuyNowBF:btn_buynowCC_LG.gif:NonHosted">
<input type="hidden" name="notify_url" value="http://example.com/test/listener.php">
<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>
Help me please with this problem.