Add the following for the HTML part of the script into register.php:
PHP:
<select name=\"gender\">
<option value=\"0\">Male</option>
<option value=\"1\">Female</option>
</select>
This should turn up as a select drop down box consisting of male and female as the choices.
Right before...
Make sure you declared the gender variable by adding this:
PHP:
$gender = $_POST['gender'];
So now, when people select male, the form will pass it as 0 so $gender would equal to 0, if its female it'll be 1.
Now under error messages, you can add your own validation for it, I suggest you add a check if its an integer and 1 character long since its only 0 or 1 not 2 digits. Once your done with that, be sure to modify the SQL statement....
Default one:
Modified one:
Be sure its in the right order (e.g. name, password, birthday, email, ip, gender).
Customize it however you want, varies on how your sql structure is.
EDIT: Edit it accordingly since the post disrupted with the formatting.