Helllo I am getting Warning: Cannot modify header information - headers already sent by error on header("location: sucessful_registration.php"); line
please help me how can i fix it

function NewUser()
{
    $full_name = $_POST['full_name'];
    $user_name = $_POST['user_name'];
    $password = $_POST['password']; 
    $email_address = $_POST['email_address'];
    $phone = $_POST['phone'];
    $address = $_POST['address'];
    $city = $_POST['city']; 
    $zip = $_POST['zip'];
    $state = $_POST['state'];
    $country = $_POST['default_country'];
    $member_id = $_POST['member_id'];
    $card_csv = $_POST['card_csv']; 
    $activation_new = "0";
    $query = "INSERT INTO user_registration (full_name,username,password,email,phone,address,city,zip,state_province,country,member_id,member_csv,activation) VALUES ('$full_name','$user_name','$password','$email_address','$phone','$address','$city','$zip','$state','$country','$member_id','$card_csv','$activation_new')";
    $data = mysql_query ($query)or die(mysql_error());
    if($data)
    {
                header("location: sucessful_registration.php");
    }
}
newuser();

    } 

I tried all the solutions but still not working

Make sure there is nothing outputted before calling header(). I'm positive you missed something, since you're not showing all your code. It could be as simple as a space somewhere.

here is the complete code

<?php
ob_start();
include('connect.php');
if (isset($_POST['formsubmitted'])) {

    $error = array();//Declare An Array to store any error message

    if (empty($_POST['full_name'])) {
        $error[] = 'Please Enter your full name';
    } else {

        if (preg_match("/^[a-z0-9 .\-]+$/i", $_POST['full_name'])) {
           //regular expression for text inputs
            $full_name = $_POST['full_name'];
        } else {
             $error[] = 'Full name is invalid';
        }


    }



        if (empty($_POST['user_name'])) {
        $error[] = 'Please enter a username';
    } else {

        $user_name = $_POST['user_name'];       
    }


    if (empty($_POST['password'])) {
        $error[] = 'Please enter a password';
    } else {

        $password = $_POST['password'];     
    }


    if (empty($_POST['phone'])) {
        $error[] = 'Please enter a phone number';
    } else {

        $phone = $_POST['phone'];       
    }



    if (empty($_POST['address'])) {
        $error[] = 'Please enter a valid address';
    } else {

        $address = $_POST['address'];       
    }



    if (empty($_POST['city'])) {
        $error[] = 'Please enter a valid address';
    } else {

        $city = $_POST['city'];     
    }



    if (empty($_POST['zip'])) {
        $error[] = 'Please enter Zip/Postal code';
    } else {

        $zip = $_POST['zip'];       
    }


    if (empty($_POST['state'])) {
        $error[] = 'Please enter State/Province';
    } else {

        $state = $_POST['state'];       
    }



    if (empty($_POST['default_country'])) {
        $error[] = 'Select your country';
    } else {

        $county = $_POST['default_country'];        
    }




    if (empty($_POST['card_csv'])) {
        $error[] = 'Please enter card CSV';
    } else {

        $card_csv = $_POST['card_csv'];     
    }



    if (empty($_POST['email_address'])) {
        $error['email_address'] = 'Please Enter your Email ';
    } else {


        if (preg_match("/^([a-zA-Z0-9])+([a-zA-Z0-9\._-])*@([a-zA-Z0-9_-])+([a-zA-Z0-9\._-]+)+$/", $_POST['email_address'])) {
           //regular expression for email validation
            $email_address = $_POST['email_address'];
        } else {
             $error[] = 'Your Email Address is invalid  ';
        }


    }


    if (empty($_POST['member_id'])) {
        $error[] = 'Please enter a member id/card number';
    } else {
        $member_card = $_POST['member_id']; 

        $query = "SELECT * FROM `card_numbers` WHERE gcard_number='$member_card'";
        $result = mysql_query($query) or die(mysql_error());

        $count = mysql_num_rows($result);
//3.1.2
        if ($count == 1){
            $member_id = $member_card;
            }else{
                //3.1.3 

            $error[] = "Please enter a valid member id/card number";

            }
        }


    if (empty($error)) //send to Database if there's no error '

    { // If everything's OK...

function NewUser()
{
    $full_name = $_POST['full_name'];
    $user_name = $_POST['user_name'];
    $password = $_POST['password']; 
    $email_address = $_POST['email_address'];
    $phone = $_POST['phone'];
    $address = $_POST['address'];
    $city = $_POST['city']; 
    $zip = $_POST['zip'];
    $state = $_POST['state'];
    $country = $_POST['default_country'];
    $member_id = $_POST['member_id'];
    $card_csv = $_POST['card_csv']; 
    $activation_new = "0";
    $query = "INSERT INTO user_registration (full_name,username,password,email,phone,address,city,zip,state_province,country,member_id,member_csv,activation) VALUES ('$full_name','$user_name','$password','$email_address','$phone','$address','$city','$zip','$state','$country','$member_id','$card_csv','$activation_new')";
    $data = mysql_query ($query)or die(mysql_error());
    if($data)
    {
                header("location: sucessful_registration.php");
    }
}
newuser();

    } 


    else {//If the "error" array contains error msg , display them


$error_show =  '';
        foreach ($error as $key => $values) {

      $error_show .='   <li>'. $values.'</li>';     




        }
$error_display = "<div class='errormsgbox'> <ol>".$error_show."</ol></div>";

    }


 }// End of the main Submit conditional.


?>

IIRC you need ob_end_clean() before header().

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