Shorten the post title in WordPress

functions.php

<?php // Don't copy the opening tag

// =========================================================================
// SHORTEN THE TITLE
// =========================================================================
function shortenTitle(){
    $thetitle = get_the_title(); 
    $getlength = strlen($thetitle);
    $thelength = 45;
    echo substr($thetitle, 0, $thelength);
    if ($getlength > $thelength) {
        echo "<span class='readmore'>...</span>"; 
    }
}