I have the code below:

<?php
session_start();
echo "\n\n";
echo "inputAction.php started";

//echo "<br/><br/>";

$name = $_POST["theUser"];
$passw = $_POST["thePassword"];

if (!strcmp($name, "myPassw") && !(strcmp($passw, "1234")) )
{
    //echo "correct";
    $_SESSION['sessionUser']=$name;
    $_SESSION['sessionPassword']=$passw;
    header("location:a.php");

}
else
{
    //echo "wrong";
    header("Location: index.php");
}

?>

Burada benim ilk girişimi echo "inputAction.php started"; sonra için session_start();

Ancak, ben bir hata var olduğunu öğrendim:

inputAction.php started
Warning: Cannot modify header information - headers already sent by (output started at /Library/WebServer/Documents/01032011/inputAction.php:4) in /Library/WebServer/Documents/01032011/inputAction.php on line 16

Ama sonra yankı açıklama, bu iyi olacak fark session_start();

<?php
session_start();
//echo "\n\n";
//echo "inputAction.php started";

//echo "<br/><br/>";

$name = $_POST["theUser"];
$passw = $_POST["thePassword"];

if (!strcmp($name, "myPassw") && !(strcmp($passw, "1234")) )
{
    //echo "correct";
    $_SESSION['sessionUser']=$name;
    $_SESSION['sessionPassword']=$passw;
    header("location:a.php");

}
else
{
    //echo "wrong";
    header("Location: index.php");
}

?>

What happened? Can somebody explain why echo makes the error?
Does that mean that it's a rule not to use echo after session_start?
Any other ways to print logs (alternative to echo) that I can use after session_start?