And then comes $wpdb-> prepare. Concerning the security, the second and third arguments are used to assign values to the placeholders.
This time, number "15" of $post_id is assigned as %d. And, "2019-7-25" which is $current_now will be inserted to %s.
This assignment depends on the version of PHP you are using, but the point is that you have to use this.
Even if there is nothing to assign, please assign in either the second or the third argument.
If you do not do this, some PHP versions may not work.
In addition, although not recommended very much, but there is also a method of emptying the double quotation of the second argument as follows.