Creating a XML document

I have the following code which is displaying an xml page as with the URL 'ourcustomersdb.php'. I need this same information to be displayed with the URL ending in 'ourcustomers.xml'. (Hope that makes sense)

The code I have works, but I need the last line of code to create the xml document:

<?php
header ("Content-Type:text/xml");//Tell browser to expect xml
include ("config/init.php");
$query = "SELECT * FROM ourcustomers"; 
$result = mysqli_query($mysqli_conn, $query) or die ("Error in query: $query. ".mysql_error()); 
//Top of xml file
$_xml = '<?xml version="1.0"?>'; 
$_xml .="<ourcustomers>"; 
while($row = mysqli_fetch_array($result)) { 
$_xml .="<ourcustomer>"; 
$_xml .="<id>".$row['id']."</id>"; 
$_xml .="<customer>".$row['customer']."</customer>"; 
$_xml .="<work_done>".$row['work_done']."</work_done>"; 
$_xml .="</ourcustomer>"; 
} 
$_xml .="</ourcustomers>"; 
//Parse and create an xml object using the string
$xmlobj=new SimpleXMLElement($_xml);
//output
print $xmlobj->asXML();
//write to a file
$xmlobj->asXML(ourcustomers.xml);
?>

The line that doesn't seem to be working is the $xmlobj->asXML(ourcustomers.xml). Can someone explain why as this is the first time I'm trying to do this so I'm not fully understanding it.

Any help would be great!