php - Not returning desired substring when strpos is used for making sure that it ends with a word only

I am using the following function for extracting substring from a description field and also make sure that it ends with a complete word only.

function smalldescription($msg)
{
    $message=substr($msg, 0, strpos($msg, ' ', 500));
    $message=$message."...";
    return $message;
}
Copy code

My problem is, if the $msg is a lenghty description, then the function is returning 500 characters finely. But if $msg is not a lengthy description, say only 20 characters, then the resultant string is as... only. Can anyone help me to solve the problem.