An example of updating values where a column is not null

only_update_if_not_null.php

<?php

// you should be escaping the values you pass to the SQL query 

if (! empty($first_name)) {
    $sql = "UPDATE `user` SET `u_first` = $first_name WHERE `id`='$uid' and u_first IS NOT NULL;";
    // execute etc ..
}

// then you will need to do the same for the other fields