Automatic PHP execution

Is there anyway to execute PHP automatically?

Basically, in the end I want to be able to accomplish a script that will copy an attribute from several (hundred) XML files, and put the attribute from each XML file into another file.

EXAMPLE:


<xml>
    <myXML attr="hi"/>
    <myXML fdsfds="bye"/>
</xml>


I want to extract the attribute ‘attr’ from over a hundred XML files, and have the attribute copied to one XML file about once a day. Anyway to automatically execute the PHP to do so, without having to call the PHP myself everyday?