Custom Excerpt

Can u pls tell me what it wrong with this excerpt code for wordpress. // Excerpt Function

function excerpt($num) {

$limit = $num+1;

$excerpt = explode(‘ ‘, get_the_excerpt(), $limit);

array_pop($excerpt);

$excerpt = implode(” “,$excerpt).” ID) .” ‘ class='”.readmore.”‘>Continue Reading »“;

echo $excerpt;

}
when i am using this it’s showing- Warning: Use of undefined constant readmore – assumed ‘readmore’ (this will throw an Error in a future version of PHP) in C:\xampp\htdocs\learningwp\wp-content\themes\mytheme\functions.php on line 71.
how do can fix the problem.