convert DATE format to UNIX TIMESTAMP in mysql

Table : info
============================
id name dob
============================
1 test1 2008-12-03 00:00:00
2 test2 2009-06-01 00:00:00
============================

$sql = "SELECT UNIX_TIMESTAMP(dob) AS birthdate FROM info WHERE 1=1";

OUTPUT :
-------------
birthdate
========
1228280400
1243828800