If you output XML content from PHP file, you need to set content-type header, so that your browser know that you are going to output XML content from PHP file instead of default content-type header (text/html).

<?php
header ("Content-Type:text/xml");
?>
<your_xml_content>
</your_xml_content>

You may know, no output (no html, echo, space etc) can occur before "header()" for header being sent to browser.

Content-type header tells browser how to handle the content. For example, if there is media file then browser will give you an option to use another application if it cannot handle that content. For *.mp3 file, it may ask you to use any applications installed on your computer that is associated with mp3 or it will tell you to download the content.
text/xml file is treated as us-ascii file if there is no charset specified. So, if your charset is different then specify that in the header as below.

header('Content-Type: application/xml; charset=utf-8');