[PHP] what's the matter?

[X]

index.php:
<form action="x.php" method="post">choice your gender
 <input type="radio" name="gender" value="male"> male
 <input  type="radio" name="gender" value="female">female
 <input type="submit">
</form>         

x.php:
<?php
if($gender=='')
 echo "choice the gender";
if($gender=="male")
 echo "you are male";
if($gender=="female")
 echo "you are female";
?>         

but at last it said that the gender had not been defined???
help me!
thank u!~