Don't "break out" of php (using the ?> <?php tags). Content outside the php tags is output directly to the browser, which is certainly not what you want in this case. (Personally, I would recommend never doing this: it's very haphazard, and too easy to run into problems. But mainly, it keeps you from developing good coding habits.)
You need to create your content first, and then pass it to [font=monospace]mail()[/font].
<?php
$to = $subscribersEmail;
$subject = "{$INFO['chatName']} New Upload";
// this string definition style is called a HEREDOC.
// you don't need to worry about single- or double-quotes at all inside.
// you can use $variables as well.
// it starts with <<<HTML (or whatever letter identifier you want)
// and ends with the matching letter identifier.
$body = <<<HTML
<p style="color: red;">Check it out, $subscribers. My HTML content goes here</p>
HTML
;
$headers = "From: admin@something-awesome.com\r\n"; // your headers need to end with newlines
$headers .= "Mime-version: 1.0\r\n"; // since you're sending HTML, add the mime-version
$headers .= "Content-type: text/html; charset utf-8\r\n"; // and content-type
mail( $to,$subject,$body,$headers ); // much cleaner!
Also note:
HTML in emails needs to be "oldskool."
linked stylesheets, classes, and many other things you may take for granted don't work in emails.
in emails, things like table-based layouts, spacer gifs, inline styles, and other "Bad Things" are still in common use - the modern alternatives aren't always an option.
