В моем случае:
header('Content-Type: application/xml;');
header('Content-Disposition: attachment; filename="strings.xml"');
//for example
echo '<?xml version="1.0" encoding="utf-8"?>';
echo '<!-- ********************** -->';
генерирует <?xml version="1.0" encoding="utf-8"?><!-- ********************** -->
С 

:
header('Content-Type: application/xml;');
header('Content-Disposition: attachment; filename="strings.xml"');
//for example
echo '<?xml version="1.0" encoding="utf-8"?>
';
echo '<!-- ********************** -->';
генерирует <?xml version="1.0" encoding="utf-8"?>
<!-- ********************** -->
Просто с командой ввода это работает:
header('Content-Type: application/xml;');
header('Content-Disposition: attachment; filename="strings.xml"');
//for example
echo '<?xml version="1.0" encoding="utf-8"?>
';
echo '<!-- ********************** -->';
генерирует
<?xml version="1.0" encoding="utf-8"?>
<!-- ********************** -->