it depends what sort of ecard you have in mind.
1) send the ecard via email, just some picture and text attached, you can achieve that with php only
2) generate more complex graphics drag&drop, position and resize graphic elements and then send as email, than I would recommend flash app for editing and php script to upload final picture to server and send the email
3) send simple card via email with generic graphics and link to a website with animated content and personalized message (flash, php, database)
difficulty increases from top of the list to the bottom, 3 being the most complex