无法在WordPress中发送多个PHP mail（）标头

I am using Gravity Forms to handle forms and after the form submission I have coded a simple mail() function to send an email to the user. The headers work fine individually:

$headers = 'From: MyName
';

// or 

$headers .= 'Content-type: text/html; charset=iso-8859-1
';

mail('me@gmail.com', 'My Subject', 'My Content', $headers);

but together in either order there is an issue"

$headers = 'From: MyName
'; // works fine
$headers .= 'Content-type: text/html; charset=iso-8859-1
'; // In this case the body is not rendered as HTML
// or
$headers = 'Content-type: text/html; charset=iso-8859-1
'; // renders as HTML
$headers .= 'From: MyName
'; // This now gives "unknown sender"

Any ideas?

Line breaks need to be double-quoted in order for " " to be respected. As the headers are currently defined, ' ' is being treated as literal text, not a line break.

$headers = "From: MyName
";
$headers .= "Content-type: text/html; charset=iso-8859-1
";