user23442: I'm here because I couldn't find any answer anywhere and the problem seems pretty easy and hard in a same time.
I have a website, for exemple : My_website.host.fr
I have arduino's sensors at home, for exemple temperature sensor in my garage, servomotor in shutter, etc... with Wifi-module.
I want to, as a User, access the website, update some information on the website, for exemple : I want the shutter down, so I put it to 0 on the website, and the arduino will, for exemple each X seconds check if the value has changed, if it does then it updates itself and so lower the shutter.
So, by this, i'll never have to connect directly through an @ip to my arduino, without hosting a server at home which could be hacked or anything, the arduino update itself with the infos that are on the website,
So from my arduino I must ? I have no idea, get the source code of the web page and search for some value, or, from the web page, update a file with PHP variables and retrieve that file with my arduino then read it for the values ?
I don't know if it's clear tho and I have no idea how to manage it.
Sorry about the bad english sentence or words, not main language;
Thank for the help i'm kind of lost.

user20245: I believe the term you want to search on is "web scraping".  Assuming you are fine with people knowing the state of "things" in your home, you can simply leave the states up on your public web page.  That is, you click on a button on your web page and change the text from "shutter up" to "shutter down".  Mean while, your Arduino is "web scraping" the same page looking for "shutter" and acting on the next word "up" or "down".
In this blog, someone has posted his Arduino "web scrapping" code.  I have not tried it, so do some research before considering this particular solution.

user313: Another option is Blynk who offer a server, a smartphone app, and a set of libraries and example sketches for Arduino. Also an active and helpful online user community, where the developers participate.
The server does not make view-able web pages; its job is to enable the Arduino and the app to communicate with each other.
The app lets the user quickly and easily configure a custom control panel which can communicate both ways with your Arduino.
The Arduino Blynk libraries can be installed and an example program compiled and uploaded a few minutes, with support for I/O port reading and writing already built-in. Online documentation describes how to expand the example code to do more specific things. Blynk is still under active development, meaning the documentation isn't always as detailed as one might like, that bugs are being fixed and occasional new ones appear, and new features are being developed.
I use it myself and teach other people how to, and I find it a really quick and easy to use platform for interactive remote control of my Arduino-based gadgets.
For example, I am using it right now to develop a ventilation controller for room- and window-fans. It will mix the room air in winter when the heat rises to the ceiling, and ventilate or just mix the room in summer depending on the floor, ceiling, and outdoor temperatures. It will report back to the smartphone which fans are on and off; allow me to turn any of them on or off; and display a history of the temperatures and fan states.

user7508: From your comment of Jul 1st, I understand that you can program your
server in PHP. Then, and to expand on my own comments, I am only
recommending that, whatever Web API you design, you keep it as simple as
possible. For example, for telling the Arduino whether it should keep
the shutter open or closed, you could write something like this in PHP:
header('Content-Type: text/plain');

if (user_wants_shutter_open())
    echo "open";
else
    echo "close";

In particular, it makes absolutely no sense to write a PHP program that
generates an HTML document. Consider that the Arduino is a very low
powered platform: there is no point in having it process more than a
handful of bytes when you only want to communicate a single bit of
information. And you should know that there is really nothing wrong with
using HTTP to transport plain text (but you should use the header
Content-Type: text/plain). If you want to send a more complex set of
data, you could use something like text/tab-separated-values, or maybe
even application/json.
Concerning web scrapping, it is a solution to a specific problem, the
problem being that the server returns the information in a format that
is not very well suited for machine consumption. You do not have this
problem. Obviously, you could create the problem by having your PHP
return an HTML document, and then you could apply the solution. My point
is that it is better to avoid creating the problem in the first place:
just return the information in a format that is as easy to parse as
possible.

23442: <p>I'm here because I couldn't find any answer anywhere and the problem seems pretty easy and hard in a same time.</p>

<p>I have a website, for exemple : My_website.host.fr
I have arduino's sensors at home, for exemple temperature sensor in my garage, servomotor in shutter, etc... with Wifi-module.</p>

<p>I want to, as a User, access the website, update some information on the website, for exemple : I want the shutter down, so I put it to 0 on the website, and the arduino will, for exemple each X seconds check if the value has changed, if it does then it updates itself and so lower the shutter.</p>

<p>So, by this, i'll never have to connect directly through an @ip to my arduino, without hosting a server at home which could be hacked or anything, the arduino update itself with the infos that are on the website,</p>

<p>So from my arduino I must ? I have no idea, get the source code of the web page and search for some value, or, from the web page, update a file with PHP variables and retrieve that file with my arduino then read it for the values ?</p>

<p>I don't know if it's clear tho and I have no idea how to manage it.</p>

<p>Sorry about the bad english sentence or words, not main language;</p>

<p>Thank for the help i'm kind of lost.</p>
 20245: <p>I believe the term you want to search on is "<a href="https://en.wikipedia.org/wiki/Web_scraping" rel="nofollow">web scraping</a>".  Assuming you are fine with people knowing the state of "things" in your home, you can simply leave the states up on your public web page.  That is, you click on a button on your web page and change the text from "shutter up" to "shutter down".  Mean while, your Arduino is "web scraping" the same page looking for "shutter" and acting on the next word "up" or "down".</p>

<p>In <a href="http://www.rogerclark.net/arduino-ethercard-web-scraping/" rel="nofollow">this blog</a>, someone has posted his Arduino "web scrapping" code.  I have not tried it, so do some research before considering this particular solution.</p>
 313: <p>Another option is <a href="http://www.blynk.cc/" rel="nofollow noreferrer">Blynk</a> who offer a server, a smartphone app, and a set of libraries and example sketches for Arduino. Also an active and helpful online user community, where the developers participate.</p>

<p>The server does not make view-able web pages; its job is to enable the Arduino and the app to communicate with each other.</p>

<p>The app lets the user quickly and easily configure a custom control panel which can communicate both ways with your Arduino.</p>

<p>The Arduino Blynk libraries can be installed and an example program compiled and uploaded a few minutes, with support for I/O port reading and writing already built-in. Online documentation describes how to expand the example code to do more specific things. Blynk is still under active development, meaning the documentation isn't always as detailed as one might like, that bugs are being fixed and occasional new ones appear, and new features are being developed.</p>

<p>I use it myself and teach other people how to, and I find it a really quick and easy to use platform for interactive remote control of my Arduino-based gadgets.</p>

<p>For example, I am using it right now to develop a ventilation controller for room- and window-fans. It will mix the room air in winter when the heat rises to the ceiling, and ventilate or just mix the room in summer depending on the floor, ceiling, and outdoor temperatures. It will report back to the smartphone which fans are on and off; allow me to turn any of them on or off; and display a history of the temperatures and fan states.</p>
 7508: <p>From your comment of Jul 1st, I understand that you can program your
server in PHP. Then, and to expand on my own comments, I am only
recommending that, whatever Web API you design, you keep it as simple as
possible. For example, for telling the Arduino whether it should keep
the shutter open or closed, you could write something like this in PHP:</p>

<pre class="lang-php prettyprint-override"><code>header('Content-Type: text/plain');

if (user_wants_shutter_open())
    echo "open";
else
    echo "close";
</code></pre>

<p>In particular, it makes absolutely no sense to write a PHP program that
generates an HTML document. Consider that the Arduino is a very low
powered platform: there is no point in having it process more than a
handful of bytes when you only want to communicate a single bit of
information. And you should know that there is really nothing wrong with
using HTTP to transport plain text (but you should use the header
<code>Content-Type: text/plain</code>). If you want to send a more complex set of
data, you could use something like text/tab-separated-values, or maybe
even application/json.</p>

<p>Concerning web scrapping, it is a solution to a specific problem, the
problem being that the server returns the information in a format that
is not very well suited for machine consumption. You do <em>not</em> have this
problem. Obviously, you could <em>create</em> the problem by having your PHP
return an HTML document, and then you could apply the solution. My point
is that it is better to avoid creating the problem in the first place:
just return the information in a format that is as easy to parse as
possible.</p>
