Add custom PHP code in Magento CMS page

1. create a new CMS page, with URL key called test

2. create a new template file called myfile.phtml:
app/design/frontend/mycustom/default/template/page/myfile.phtml

3. put this line in the CMS page:
{{block type="core/template" template="page/myfile.phtml"}}

4. access your page: http://mysite.com/test