Link value of form in PHP - php

Question

I have an html file, basically a simple form: The purpose is to submit a value that runs a piece of code on a PHP file ('alternative.php') See sample of html code:
<form name="input" action="alternative.php" method="POST">
Area: <input type="text" name="area"><br><br>
<input type="submit"><br>
</form><br><br>
This runs smoothly
Now I have a second PHP file ('alternative2.php') and this file automatically needs to link to the data that is input in the form.
Excerpt of php code for alternative2:
<?php
require_once 'header.php';
/** Create HTTP POST */
$accomm = 'ACCOMM';
$region = '';
foreach ($result->area as $entry) {
$region = $entry->attributes()->area_name;
break;
}
$page = '10';
Both alternative.php and alternative2.php require header.php.
Excerpt of header.php:
<?php
/** Create HTTP POST */
$country = 'Australia';
$area = htmlspecialchars($_POST["area"]);
$seek = '<parameters>
<row><param>COUNTRY</param><value>'. $country .'</value></row>
<row><param>AREA</param><value>'. $area .'</value></row>
</parameters>';
Currently it returns "Notice: Undefined index: area in C:\xampp\htdocs...." when I run it.
How do I go about this?
Thanks

Answer 1

The error suggest that $_POST["area"] is not defined, if you don't reach alternative2 from your form, then that's why you see this; if you want to reach alternative2 from other place (for example directly), or if the value of a variable must be the same on several pages, then you may want to consider using Sessions.

Answer 2

Check if the $_POST values exist before using them...
if (isset($_POST["area"])){
//do stuff
}

Answer 3

Basically it has to do with which file your form actually submits to. If your form submits to only alternative.php then you aren't receiving the POST information to your second page. The easiest and logical choices in my opinion (based on what I see from your code) is to merge the functionality of alternative.php and alternative2.php into a single page, or use sessions to store the POST information which will then be available to both pages. If you were to use sessions you would be doing something like shown below.
Start with the file that handles your form input (alternative.php I presume) and add
session_start();
to the top of that file. Then, in whatever block of code you have getting your form information add the following line:
$_SESSION['area'] = $_POST['area'];
Now your information is stored and will be available from request to request.
Then in your head.php file, access the info via $_SESSION variables.
<?php
/** Start Session */
session_start();
/** Create HTTP POST */
$country = 'Australia';
$area = htmlspecialchars($_SESSION['area']); //Access your session variable.
$seek = '<parameters>
<row><param>COUNTRY</param><value>'. $country .'</value></row>
<row><param>AREA</param><value>'. $area .'</value></row>
</parameters>';
?>