Cannot modify header information headers already sent-PHP user authentication.

Once, I was writing a PHP user authentication script and run into the same problem. My login file included connect.inc file and down the login.php file I used cokies to store a user name, password and a role. I knew that when I call setcookie() function, a header is sent. I knew that, a header is sent also when page has any ouput.

My connect.inc file did not have any output, and '<?' tag started right away from the top of the page and there was no space characters I was aware of, however, I kept getting messages: Cannot modify header information - headers already sent by...connect.inc"

<?php

include(´../connect.inc´)

//data comes from login form when user enters login and password
$username = $_POST['username'];
$password = $_POST['password'];

//encript password:
$password=md5($password);

$num=0;

$auth = false; // Assume user is not authenticated yet

       //Check if login and password are not empty
if (isset( $username ) && isset($password))
{
       //Connect to MySQL
       mysql_connect( $hostname, $dbuser, $dbpassword)
       or die ( ´Unable to connect to server. (login)´ );
       mysql_select_db($dbname )
       or die (´Unable to select database.(login)´);

//for Cyrillic characters (I had russian web site)
mysql_query("SET NAMES ´cp1251´");

// Check if username and password exist in the database
//and if the user is activated.

     $sql = "SELECT username, password, role FROM usernames WHERE
       username = ´".$username."´ and
       password = ´".$password."´ and active=1";

$result = mysql_query( $sql ) or die ( ´Unable to execute query.´ );
$num = mysql_numrows( $result );

       if ( $num == 1 )
       {
       $auth = true;
       $row=mysql_fetch_row($result);

          $auser=$row[0];
          $apassword=$row[1];
          $arole=$row[2];
          setcookie("user",$auser);
          setcookie("role",$arole);
          setcookie("password", $apassword);
       }

if (!$auth)
{ //Check if user exists but not active, then send email to the user.
$asql = "SELECT active, email FROM usernames WHERE username = '".$username."' AND password = '".$password."'";

$aresult = mysql_query( $asql ) or die
( "Unable to execute query.");

     if(mysql_numrows( $aresult ))
     {
          $arow=mysql_fetch_row($aresult);
          $active=$arow[0];
          $email=$arow[1];

          if($active == 0)
          { //Preparation parameters for mail function.
       $subject = 'User Activation';
       $headers = 'MIME-Version: 1.0' . "\r\n";
       $headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
       $headers .='From: webmaster@mydomain.com' . "\r\n";
       $parameter='-fwebmaster@mydomain.com';

$message =´Click this link to activate your account<br/>´;

$message.=´http://www.mydomain.com/activate.php?username=´.$username;

//You must wrap message text for mail.
$message = wordwrap($message, 70);

       mail($email, $subject, $message, $headers, $parameter);
$activatemessage="Your account is not active. Please check your email and activate your account.";
       setcookie("msg",$activatemessage);
       header("Location:sign_in.php");
       }
     }
     else
     {
       $loginmessage="Your login or password is not correct!";
        setcookie("msg",$loginmessage);
       header("Location:sign_in.php");
     }
   }
   else //if no aresult
   {
       setcookie("msg","");
       header("Location:display_mypprofile.php");
   }
}
else //if username and password are not set
{
$nodata="Please enter username and password!";
setcookie("msg",$nodata);
header("Location:sign_in.php");
}

When I tested my login script, I was getting this famous message "Cannot modify header information - headers already sent by connect,inc file. I looked at my connect.inc file content and could not undestant what could be wrong with such a simple file?

I searched internet, read forum posts, but could not find the exact answer. Then I opened the file in notepad, placed my mouse cursor on the last record in the file and started to press -> key on my keyboard. To my surprise, I discovered that my cursor was moved beyond the ?> tag and all the way to the end of the line. I deleted space characters after ?> tag, save the file, uploaded it and message disappeared!

Check also if you have space characters before <?php tag. And of cause you should not have any echo or print commands or calls to seccion function.

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Cannot modify header information headers already sent-PHP user authentication.