user26818: I am using an Arduino Mega and I have a basic webpage built using EthernetServer for the connection and EthernetClient to write the HTML to the client. I have a simple web form:
client.println("<form method=get>");
client.println("<input type=text name=data>");
client.println("<input type=submit value=Submit>");
client.println("</form>");

I'd like to receive any input from the form and check it against values in order to do stuff on the back end (e.g. reset certain data). How do I get the data from the web page?

user13594: Your form method is set to GET. In this case, when Submit is clicked, the client/browser sends request with values in URI:
GET /index.html?data=user_value&default_name_of_button=Submit HTTP/1.1
...more lines to follow...

These HTTP header lines will be available on your EthernetClient instance, retrieve them line-by-line using connected(), available(), read() methods. Then parse the line, find the user_value.
Please note that browser retrieves your page (to display it) using GET method too, in that case URI will be like:
GET /index.html HTTP/1.1
...more lines to follow...

It is recommended to use POST method to submit form values. Example content section (before content, server must send HTTP header - check Arduino Web Server example)
<form method="POST" action="foo.php">

First Name: <input type="text" name="first_name" /> <br />
Last Name: <input type="text" name="last_name" /> <br />

<input type="submit" name="action" value="Submit" />

</form>

In this case the values are returned in content section:
POST /foo.php HTTP/1.1
Host: localhost
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5 (.NET CLR 3.5.30729)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
Referer: http://localhost/test.php
Content-Type: application/x-www-form-urlencoded
Content-Length: 43

first_name=John&last_name=Doe&action=Submit

Tutorial on HTTP headers:
https://code.tutsplus.com/tutorials/http-headers-for-dummies--net-8039

26818: <p>I am using an Arduino Mega and I have a basic webpage built using <code>EthernetServer</code> for the connection and <code>EthernetClient</code> to write the HTML to the client. I have a simple web form:</p>

<pre><code>client.println("&lt;form method=get&gt;");
client.println("&lt;input type=text name=data&gt;");
client.println("&lt;input type=submit value=Submit&gt;");
client.println("&lt;/form&gt;");
</code></pre>

<p>I'd like to receive any input from the form and check it against values in order to do stuff on the back end (e.g. reset certain data). How do I get the data from the web page?</p>
 13594: <p>Your form method is set to <code>GET</code>. In this case, when <code>Submit</code> is clicked, the client/browser sends request with values in URI:</p>

<pre><code>GET /index.html?data=user_value&amp;default_name_of_button=Submit HTTP/1.1
...more lines to follow...
</code></pre>

<p>These HTTP header lines will be available on your <code>EthernetClient</code> instance, retrieve them line-by-line using connected(), available(), read() methods. Then parse the line, find the <code>user_value</code>.</p>

<p>Please note that browser retrieves your page (to display it) using <code>GET</code> method too, in that case URI will be like:</p>

<pre><code>GET /index.html HTTP/1.1
...more lines to follow...
</code></pre>

<p>It is recommended to use <code>POST</code> method to submit form values. Example content section (before content, server must send HTTP header - check <a href="https://www.arduino.cc/en/Tutorial/WebServer" rel="nofollow noreferrer">Arduino Web Server</a> example)</p>

<pre><code>&lt;form method="POST" action="foo.php"&gt;

First Name: &lt;input type="text" name="first_name" /&gt; &lt;br /&gt;
Last Name: &lt;input type="text" name="last_name" /&gt; &lt;br /&gt;

&lt;input type="submit" name="action" value="Submit" /&gt;

&lt;/form&gt;
</code></pre>

<p>In this case the values are returned in content section:</p>

<pre><code>POST /foo.php HTTP/1.1
Host: localhost
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5 (.NET CLR 3.5.30729)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
Referer: http://localhost/test.php
Content-Type: application/x-www-form-urlencoded
Content-Length: 43

first_name=John&amp;last_name=Doe&amp;action=Submit
</code></pre>

<p>Tutorial on HTTP headers:</p>

<p><a href="https://code.tutsplus.com/tutorials/http-headers-for-dummies--net-8039" rel="nofollow noreferrer">https://code.tutsplus.com/tutorials/http-headers-for-dummies--net-8039</a></p>
