tags in a PHP document is written to the response body. There is something before the very first line of your myfile.php. You have probably saved your source file as UTF-8 with a byte order mark (BOM) and your declared encoding is something like ISO 8859-1. Remove the leading 3 bytes of “EF BB BF” from your source file with a binary editor. For example the Visual Studio has a well hidden editor: File -> Open File -> choose your file -> click on the Open button’s selection bar -> Open With -> Select binary editor -> Open.
<?php ... ?>