You have a syntax error. Just remove the single quotes in the href
:
$message->setBody(
'<html>' .
' <head></head>' .
' <body>' .
' This is my message' .
'<a href="http://www.mazihealth.com">Sign in</a>'.
' </body>' .
'</html>',
'text/html' // Mark the content-type as HTML
);
Additionally, you might find it easier to use heredoc statements to outline your content. It makes reading/editing large strings easier. Here is an example:
$body = <<<EOD
<html>
<head></head>
<body>
This is my message .
<a href="http://www.mazihealth.com">Sign in</a>
</body>
</html>
EOD;
$message->setBody(
$body,
'text/html' // Mark the content-type as HTML
);