Code changes in PHP files without edits

Within many of my php webpage files,  the code (this is one example) has changed from <form action="../mailer.php" method="POST" enctype="multipart/form-data" autocomplete="off" to <form action="file:///Macintosh HD/Users/user_name/Desktop/Imagine/mailer.php" method="POST" enctype="multipart/form-data" autocomplete="off" (note: the user_name is my system User Name)

I have not modified the files to alter this, it just happens.

Any thoughts as to why this is occurring and what I can do to ensure it does not reoccur inthe future?