HTML Email Sent Via PHP Treated As Spam In Gmail

I have the following php script which loads a html template from a .txt file on my server and uses str_replace to populate the template with the Email content before sending the em

Solution 1:

Theres a million reasons why gmail would suspect the email is spam.

The first thing I would do is use a library that deals with headers correctly -> http://swiftmailer.org/

Secondly, ensure your sender host is coming from the same IP as the server and have correct MX records.