Hi All

Please somehelp me to correct my code so that after the user click the submit button it redirects to the correct URL if the value is correct. I get the error when pressing submit this php file will be displayed in an iframe
Error
Warning: Cannot modify header information - headers already sent by

Code:
        <?php

        function test_input($data) {
            $data = trim($data);
            $data = stripslashes($data);
            $data = htmlspecialchars($data);
            return $data;
        }

        $amountErr = "";
        $amount = "";
        if ($_SERVER["REQUEST_METHOD"] == "POST") {
            if (($_POST["amount"]) < 250) {
                $amountErr = "Value must be equal or greater than 250";
            } else {
                $amount = test_input($_POST["amount"]);
                header('location:https://www.moneybookers.com/app/payment.pl');
            }
        }
        ?>
      

        <form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>"
        
            <p><span class="error">*required field.</span></p>
            Amount:<input type="text" name="amount"/>
            <span class="error">*<?php echo $amountErr; ?></span><bR>
            
             <input type="hidden" name="pay_to_email" value="someone.com"/>
             <input type="hidden" name="status_url" value="someone.com"/>
             <input type="hidden" name="return_url" value="http://wwww.somesite.com"/>
             <input type="hidden" name="language" value="EN"/>
             <input type="hidden" name="currency" value="USD"/>
             <input type="hidden" name="detail1_description" value="New Deposit"/>
             <input type="hidden" name="detail1_text" value="New Deposit"/><BR>
             <input type="submit" value="Deposit!"/>