paypal and Php?

I have two ways to check from user:

1.if user input (Amount field < 5(user credit):
   do update database the remain amount in my database table.
2.if user input(Amount field) > 5(user credit):
  Do paypal transaction with the submit form.



 <form action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post" id="payPalForm">        
    <input type="hidden" name="item_number" value="01 - General Payment to FreelanceSwitch.com">
    <input name="item_name" type="hidden" id="item_name"  size="45" value="Posting job">    
    <input type="hidden" name="cmd" value="_xclick">    
    <input type="hidden" name="no_note" value="1">    
    <input type="hidden" name="business" value="seller_1265789181_biz@xxx.xxx.xx">    
    <input type="hidden" name="currency_code" value="USD">    
    <input type="hidden" name="return" value="http://freelanceswitch.com/payment-complete/">       
    Paying Page:<input name="amount" type="text" id="amount" size="5">   
    <input type="submit" name="Submit" value="Pay">            
    </form开发者_运维知识库>

Anybody could tell me how to do with the paypal submit form with the conditions.?

thanks.

Thanks @Donny Kurnia ,now I have tried:

EDIT::

function compare_user_credit($paying_price, $user_credit){

    $db = &JFactory::getDBO();
    $user =& JFactory::getUser();
    $user_id = $user->get('id');

    if ($paying_price <= $user_credit) {        
        $remain_credit = $user_credit - $paying_price;
        //DO UPDATE:enough credit
        update_user_credit_after_paying($remain_credit,$user_id);
        $action_after_paying = header("Location: index.php?xxxx=5");
     }
    # Case2 direct to paypal with the submit form.
    else if ($paying_price > $user_credit){
       //Need submit form to paypal.
       $action_after_paying = header("Location: index.php=xxxx&paypal=".$_POST["amount"]);
    }
    return $action_after_paying;
}
$output = '<form method="POST" > ';
     $output .='<h1>Credit:'.get_credit().'</h1>';
     $output .= ' Paying Page <input type="text" name="amount" size=3 max_length=5 />';
     $output .='<input type="submit" name="pay" value="pay" />' ;
     echo $output;

if(isset($_POST["pay"])){
    compare_user_credit($_POST["amount"], $user_credit);

}

HOw Could I do with Paypal in Case 02.(direct to paypal with submit form.)

Maybe you can receive the form submit to your own code and then check it's value. If it satisfy the first condition, do the database update. If it satisfy the second condition, redirect the user to a page that have hidden form that will submit to paypal. You can populate the data in this hidden form using data from database or previous form input, and put a javascript code so the form will automatically submitted when the page is loaded.

In my code, I use Micah Carrick's Paypal IPN class to send data to paypal. The class have a sample code on how to display the hidden form, with a submit button in case the javascript is disabled.