Update an XML file using a PHP script

I'm making an interface-website to update a concert-list on a band-website.
The list is stored as an XML file an has this structure :

<?xml version='1.0' encoding='ISO-8859-1'?>
<gigs><gig id="1" day="01" month="01" year="2000" venue="New York" /></gigs>
<gigs><gig id="2" day="02" month="01" year="2000" venue="Berlin" /></gigs>

I already wrote a script that enables me to add a new gig to the list, this was relatively easy...
Now I want to write a script that enables me to edit a certain gig in the list.
Every Gig is Unique because of the first attribute : "id" .
I want to use this reference to edit the other attributes in that Node.

My PHP is very poor, so I hope someone could put me on the good foot here...

My PHP script :

<?php
$id = $_POST['id'];
$day = $_POST['day'];
$month = $_POST['month'];
$year = $_POST['year'];
$venue = $_POST['venue'];

$xml_file="gig.xml";
$fh = fopen($xml_file,'r');
$current_tag=" ";
function start_element($parser,$element_name,$element_attrs)
{
      $current_tag = $element_name;
      if($element_name == 'GIG' && $element_attrs["ID"] == $id)
      {
            echo 'gig ID =' . $id;
                                             // here the new info has to replace the old
      };
};
if($fh) {  
echo "&verify=success&";
} else {  
echo "&verify=fail&";
}
fclose($fh);
?>


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