为什么我的代码显示已经发送的标题，即使它处于if else条件下？ [重复]

Question

This question already has an answer here:

• How to fix “Headers already sent” error in PHP 11 answers

I have a very simple code which checks whether email address exists in the table or not. If it exists then alert Already Exists and redirect to another page and if it doesn't exist then insert into table. Inserting when the email address is not present in the table is happening properly and is getting redirected to the respected page but when the email is existing in the table then although it alerts that username exists, it shows headers already sent at error

My code is

<?php   
    require_once('connection.php'); 
    $tbl_name="cust_contacts";      
        $ccode=$_POST['ccode'];
        $name=$_POST['name'];
        $des=$_POST['desig'];
        $dep=$_POST['dep'];
        $phone=$_POST['phone'];
        $mobile=$_POST['mob'];
        $email=$_POST['email']; 
        $check="select * from $tbl_name where email='$email'";
        $checkqry=mysql_query($check);
        if($checkqry['error'])
            die(mysql_error()); 
        $countcheck=mysql_num_rows($checkqry);
        if($countcheck==0)
        {
            $qry="insert into $tbl_name(cust_code,name,designation,department,phone,mobile,email) values('$ccode','$name','$des','$dep','$phone','$mobile','$email')";
            if (!mysql_query($qry))
            {
                die('Error: ' . mysql_error());
            }       
            header("location:index.php?customername=$ccode#pop4");
        }
        else
        {
            echo '<script type="text/javascript">alert("User already exists. Try editing the user");</script>';
            header("location:index.php?customername=$ccode");
        }                   
    //$count=mysql_num_row($result);
    //if($count==1) 
?>

</div>

Answer 1

If you want to show an alert message and redirect to another page, you can use JavaScript like this:

Befor (Your Original Code):

    echo '<script type="text/javascript">alert("User already exists. Try editing the user");</script>';
    header("location:index.php?customername=$ccode");

After:

 echo '<script type="text/javascript">';
 echo  'alert("User already exists. Try editing the user");';
 echo  'location.href = "index.php?customername=' . $ccode. '";';
 echo '</script>';

Hope this helps.