You can not call a phtml file by writing PHP code in Magento admin panel as Magento admin panel do not support this. To enable PHTML/ PHP code you can write a file called PHTML file by doing the following method
{{ block type="core/template" name="mycstompage" template="SkyMagento/mycustompage.phtml" }}
inside template = "" allows you to write your path. and in the name = "", you can give a good and unique name type = "", you can also write your module directory (for example: - the client / account if you call login.phtml file).
Another way to call a phtml file. You can use the layout or xml file to do so. I'm going mycstompage.phtml attached file that reside in the folder of my mycustompage template inside SkyMagento folder then I write the code given below
<Reference name="content"> <Block type="core/template" name="mycustompage" template="SkyMagento/mycustompage.phtml" /> </ Reference >