Having problems with header PHP function

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PHP

I am having some trouble using the header function in php. My code is supposed to check several Boolean values and execute the appropriate block of code. When I execute the script I get this error message,"Warning: Cannot modify header information - headers already sent by (output started at /home2/truck35/public_html/ShowLove/Captcha_Post.php:34) in /home2/truck35/public_html/ShowLove/Captcha_Post.php on line 73". I read that no characters are supposed to be outputted before the function call but, I am a little confused what is actually meant by this. The script works on my development WAMP server. Any help would be appreciated. Here is the script that I am working on.

<?php
session_start();
require_once('Test_Input.php');


if(!isset($_SESSION['Form_First_Name']) || !isset($_SESSION['Form_Last_Name']) || !isset($_SESSION['Form_Email']) || !isset($_SESSION['Form_Message']))
{
	$_SESSION['Form_First_Name'] = '';
	$_SESSION['Form_Last_Nameorm'] = '';
	$_SESSION['Form_Email'] = '';
	$_SESSION['Form_Message'] = '';
} 


if(isset($_POST['Clear']) && $_POST['Clear'] == 'clear')
{
	$_SESSION['Form_First_Name'] = '';
	$_SESSION['Form_Last_Nameorm'] = '';
	$_SESSION['Form_Email'] = '';
	$_SESSION['Form_Message'] = '';
	unset($_SESSION['name_Switch']);
	unset($_SESSION['Form_Email_Switch']);
	//header('location:Contact_Form.php');
}

$response = $_POST['Response'];
$image_request = $_POST['DB_Response'];
$first_name = $_SESSION['Form_First_Name'] = $_POST['First_Name'];
$last_name = $_SESSION['Form_Last_Name'] = $_POST['Last_Name'];
$email = $_SESSION['Form_Email'] = $_POST['Email'];
$_SESSION['Form_Message'] = $_POST['Message'];


$first_name_result = Test_Input::name_validation($first_name);
var_dump($first_name_result);
$last_name_result = Test_Input::name_validation($last_name);
$email_result = Test_Input::email_validation($email);

if(!$first_name_result || !$last_name_result )
{
	$_SESSION['Name_Switch'] = false;
	header('location:Contact_Form.php');
}

if (!$email_result)
{
	$_SESSION['Form_Email_Switch'] = false;
	header('location:Contact_Form.php');
}

if($response != $image_request)
{
	 
	//unset($_SESSION['Form_Switch']);
	unset($_SESSION['Email_Switch']);
	header('location:Contact_Form.php');
}

else 
{
	global $first_name;
	global $last_name;
	global $email;
	$message = 'FROM: '.$first_name.' '.$last_name.' \n\n'. $_POST['Message'];
	$to = 'admin@showlove-productions.com';
	$subject = 'Show Love Production Contact Form';
	$headers = $email;
	mail($to, $subject, $message, $headers);
	$_SESSION['Email_Switch'] = true;
	unset($_SESSION['Form_First_Name']);
	unset($_SESSION['Form_Last_Name']);
	unset($_SESSION['Form_Email']);
	unset($_SESSION['Form_Message']);
header('location:Contact_Form.php');
}
?>

What I have tried:

I tried using the ob_start() and ob_end_clean() functiona for output buffering but, they do not work.