PHP - Problemas con formularios

<?php

$firstname=((isset($_POST["firstname"])&&$_POST["firstname"]!="")?htmlentities($_POST["firstname"]):"Your FirstName");

$lastname=((isset($_POST["lastname"])&&$_POST["lastname"]!="")?htmlentities($_POST["lastname"]):"Your LastName");

$sex=((isset($_POST["sex"])&&$_POST["sex"]=="1")?1:0);

?>

<form method="post" action="">

First name:<br>

<input type="text" name="firstname" value="<?=$firstname?>">

<br>

Last name:<br>

<input type="text" name="lastname" value="<?=$lastname?>">

<br>

<br>

<input type="radio" name="sex" value="0" <?=((!$sex)?"checked":"")?>>Male

<br>

<input type="radio" name="sex" value="1" <?=(($sex)?"checked":"")?>>Female

<input type="submit" value="Submit">

</form>