I want to add a verification code to my PHP website, a sliding one

How dare you think about it.

Web page verification code generally generates a verification code in the background, generates a good picture in the background and returns it to the web page for display. The information entered by the user is verified against the information saved in the back end.

If the back-end information is returned to the front-end in text form, it will not be safe.

Your function can be designed like this, but it will not be very useful. Let me talk about my idea

First, the backend returns a numeric verification code, and the frontend obtains the verification of the numeric row and uses js+css to organize and implement special effects. You set a blocking html element in the black part of the picture above (the distance between this element and the triangular slider on the left is the number returned by the backend), and how far the slider on the left slides to reach this hidden block, get this value, Save it.