I want to know how to use php variable in xml file ?

Question

Hi friends, I want to use php variable in xml file like as below

<field name="catid"
            type="hidden"
            label="JCATEGORY"
            description="JFIELD_CATEGORY_DESC"
            class="inputbox"
            default="<?php echo $a; ?>"
 />

but this php code not support in this xml i.e. no any output ? So please tell me anyone how to use php variable in xml file ? Please help ...



Thanks