Radio buttons are indicated by stand-alone input tags with attributes type="radio" and a name, like inname="gender". They also can (and must, if you submit this to a PHP page) have a value attribute, like in value="female". Note that radio buttons that belong to a group where only one is supposed to be clicked must have the same name. The word "checked" without quotation marks inside the tag (which is also an attribute) would indicate that this option is checked by default, like in <input type="radio" name="gender" value="female" checked > You can also use the XML compatible writing checked="checked".
The php-page where the data is send to obtains a variable whose name is the name of the radio button with a $ in front of it, like in $gender. The value of that variable is the value of the selected button; in the example $gender=="female" would be true.
Erich Prisner, November 2006