Why is my .htaccess file disabling my PHP email form?

Hi everyone,

I'm quite new to PHP, and I've recently reconfigured my website to use global PHP headers and footers. This involved changing all my pages from the .html extension to .php.

I have included code in the .htaccess to remove the .php from the URL of my pages, but for some reason this now results in my PHP email form not working. Instead of processing a message sent through the form, the result returns the $feedback_page when the submit button is pressed.

Here is my .htaccess code:

Options +FollowSymLinks -MultiViews
# Turn mod_rewrite on
RewriteEngine On
RewriteBase /

## don't touch /forum URIs
RewriteRule ^forums/ - [L,NC]

## hide .php extension snippet

# To externally redirect /dir/foo.php to /dir/foo
RewriteCond %{THE_REQUEST} ^[A-Z]{3,}\s([^.]+)\.php [NC]
RewriteRule ^ %1 [R,L]

# To internally forward /dir/foo to /dir/foo.php
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.*?)/?$ $1.php [L]

# To remove www header
RewriteCond %{HTTP_HOST} ^www\.(.+)$ [NC]
RewriteRule ^(.*)$ http://%1/$1 [L,R=301]

ErrorDocument 404 /404.php

and here is the send_mail.php code:

<?php
/*
This first bit sets the email address that you want the form to be submitted to.
You will need to change this value to a valid email address that you can access.
*/
$webmaster_email = "removed_email@removed.com";

/*
This bit sets the URLs of the supporting pages.
If you change the names of any of the pages, you will need to change the values here.
*/
$feedback_page = "contact.php";
$error_page = "error_message.php";
$thankyou_page = "thanks.php";

/*
This next bit loads the form field data into variables.
If you add a form field, you will need to add it here.
*/
$email_address = $_REQUEST['email_address'] ;
$comments = $_REQUEST['comments'] ;

/*
The following function checks for email injection.
Specifically, it checks for carriage returns - typically used by spammers to inject a CC list.
*/
function isInjected($str) {
	$injections = array('(\n+)',
	'(\r+)',
	'(\t+)',
	'(%0A+)',
	'(%0D+)',
	'(%08+)',
	'(%09+)'
	);
	$inject = join('|', $injections);
	$inject = "/$inject/i";
	if(preg_match($inject,$str)) {
		return true;
	}
	else {
		return false;
	}
}

// If the user tries to access this script directly, redirect them to the feedback form,
if (!isset($_REQUEST['email_address'])) {
header( "Location: $feedback_page" );
}

// If the form fields are empty, redirect to the error page.
elseif (empty($email_address) || empty($comments)) {
header( "Location: $error_page" );
}

// If email injection is detected, redirect to the error page.
elseif ( isInjected($email_address) ) {
header( "Location: $error_page" );
}

// If we passed all previous tests, send the email then redirect to the thank you page.
else {
mail( "$webmaster_email", "Contact Form - MyWebsite.com",
  $comments, "From: $email_address" );
header( "Location: $thankyou_page" );
}
?>

Any help would be much appreciated, thanks everyone.