Limit a string using substr to 30 characters.

Hey, I’m trying to use the following to limit my post topics to 30 characters, but it doesn’t see to work with the function that retrieves the data.

<?php
$post-string = the_title();
substr($post-string, 0, 30);
?>

Does anyone know how to do this? I would like it to appear like the following:

“This is a post topic exam…”
“Here is another post that…” // Cuts off after so many characters and puts three dots.

Thanks in advance to any advice!