php mail not sending once I add headers

Why does the below code not send my email? When I remove the "headers" variable from the mail function, it sends right away with no problems.

My $to and $subject and $html vars are set further up but as I said, it works fine as soon as I remove the $headers

I'm trying to send html encoded email.

// To send HTML mail, the Content-type header must be set
$headers  = 'MIME-Version: 1.0' . "\r\n";
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";

// Additional headers
$headers .= 'To: Mary <$to>' . "\r\n";
$headers .= 'From: Birthday Reminder <me@somewhere.com>' . "\r\n";

mail($to,$subject,$HTML,$headers);

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