please someone with knowledge in php for arduino that can help me do the second page to change the ip of the arduino, please help me.
There are no experts on php for Arduino, because php doesn't run on the Arduino.
my client enters my site.
Which the Arduino is serving, or not? If the Arduino serves the page, it doesn't contain php code, does it? If the Arduino is not the server, then, I don't see any easy way of changing it's IP address using browser code. The Arduino would have to issue a GET request to get the data, and it would have no way of knowing that there was a need to do so.