Update only Xml node (PHP5)

Hi all

can I update a xml node AND write the new value in a file witout
re-write all the file ?

sample :

my xml file :

<sample>
<exo id="1" value="Test" />
</sample>

i want to modify the file like

<sample>
<exo id="1" value="Test updated" />
</sample>

is it possible ?

.