Write ip to arduino from web site (PHP).

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. I stopped in time ....
I have a code that changes the ip put it runs from within the arduino. but it is not what I need.

what I'm wanting to do is the following:
my client enters my site.
its authentication will be a ddns previously registered.
and the password is personal.
Once authenticated, the valid ip (fixed) directs the modem to the local ip and port arduino.
then open the page of your home where it will control gates and symptoms (or any device 110v).

why the page setup?
because the arduino go with ip 192.168.1.25 mask 255.255.255.0 manufactures and gateway 192.168.1.254.
the customer will enter the setup of the modem
ex. 192.168.254.254
Your local band would soon 192.168.254.0/24
then he will choose a local ip that is not being used and direct a valid ip port for this ip:
eg 189.69.69.69 dst-nat tcp, udp port 8000 ip 192.168.254.25 to local port 80
made the setting on the client vai cadastar ddns for access (which will be on site)
ex
exautomation.ddns.com.br = 189.69.69.69:8000 192.168.254.25:80 = ready is already on the internet.
the customer will change your ip in 192.168.1.2 mask 255.255.255.0 to computardor
open the browser enter the ip 192.168.1.25 arduino enter the ip setup tab and put the ip and port mascara gatwey he configured the modem for internet.

Ready your home is already online.

the page with the relay is already in place and configured in arduino.
the ddns is already running,
now lack a solution for my client write ip arduino according to your local network. but everything has to be via browser.

enables someone to help me?

Thank you.