Warning: Cannot modify header information - headers already sent by (.../php_includes/db_conx.php:1) in .../loop/activation.php on line 39

Warning: Cannot modify header information - headers already sent by
(.../php_includes/db_conx.php:1) in .../loop/activation.php on line 39

What is the problem on this error?
Warning: Cannot modify header information - headers already sent by
(output started at
/home4/iamden83/public_html/loop/php_includes/db_conx.php:1) in
/home4/iamden83/public_html/loop/activation.php on line 39
and here the code:
<?php
if (isset($_GET['id']) && isset($_GET['u']) && isset($_GET['e']) &&
isset($_GET['p'])) {
// Connect to database and sanitize incoming $_GET variables
include_once("php_includes/db_conx.php");
$id = preg_replace('#[^0-9]#i', '', $_GET['id']);
$u = preg_replace('#[^a-z0-9]#i', '', $_GET['u']);
$e = mysqli_real_escape_string($db_conx, $_GET['e']);
$p = mysqli_real_escape_string($db_conx, $_GET['p']);
// Evaluate the lengths of the incoming $_GET variable
if($id == "" || strlen($u) < 3 || strlen($e) < 5 || strlen($p) != 74){
// Log this issue into a text file and email details to yourself
header("location: message.php?msg=activation_string_length_issues");
exit();
}
// Check their credentials against the database
$sql = "SELECT * FROM users WHERE id='$id' AND username='$u' AND
email='$e' AND password='$p' LIMIT 1";
$query = mysqli_query($db_conx, $sql);
$numrows = mysqli_num_rows($query);
// Evaluate for a match in the system (0 = no match, 1 = match)
if($numrows == 0){
// Log this potential hack attempt to text file and email details to
yourself
header("location: message.php?msg=Your credentials are not matching
anything in our system");
exit();
}
// Match was found, you can activate them
$sql = "UPDATE users SET activated='1' WHERE id='$id' LIMIT 1";
$query = mysqli_query($db_conx, $sql);
// Optional double check to see if activated in fact now = 1
$sql = "SELECT * FROM users WHERE id='$id' AND activated='1' LIMIT 1";
$query = mysqli_query($db_conx, $sql);
$numrows = mysqli_num_rows($query);
// Evaluate the double check
if($numrows == 0){
// Log this issue of no switch of activation field to 1
header("location: message.php?msg=activation_failure");
exit();
} else if($numrows == 1) {
// Great everything went fine with activation!
header("location: message.php?msg=activation_success");
exit();
}
} else {
// Log this issue of missing initial $_GET variables
header("location: message.php?msg=missing_GET_variables");
exit();
}
?>