Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent [duplicate]

Question

I received an error on my php code something like this:

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home4/iamden83/public_html/loop/signup.php:1) in /home4/iamden83/public_html/loop/signup.php on line 2

And the code is this

<?php
session_start();
// If user is logged in, header them away
if( isset( $_SESSION["username"] ) ){
    header( "location: message.php?msg=NO to that weenis" );
    exit();
}
?><?php
// Ajax calls this NAME CHECK code to execute
if(isset($_POST["usernamecheck"])){
    include_once("php_includes/db_conx.php");
    $username = preg_replace('#[^a-z0-9]#i', '', $_POST['usernamecheck']);
    $sql = "SELECT id FROM users WHERE username='$username' LIMIT 1";
    $query = mysqli_query($db_conx, $sql); 
    $uname_check = mysqli_num_rows($query);
    if (strlen($username) < 3 || strlen($username) > 16) {
        echo '<strong style="color:#F00;">3 - 16 characters please</strong>';
        exit();
    }
    if (is_numeric($username[0])) {
        echo '<strong style="color:#F00;">Usernames must begin with a letter</strong>';
        exit();
    }
    if ($uname_check < 1) {
        echo '<strong style="color:#009900;">' . $username . ' is OK</strong>';
        exit();
    } else {
        echo '<strong style="color:#F00;">' . $username . ' is taken</strong>';
        exit();
    }
}

Answer 1

hmmm, i think there must be some ascii or white-space character hiding before <?php , just open notepad, copy content from session_start(); to the end and paste it, now manually add <?php, and make sure there aren't any white space characters before <?php at line no 1