Use something like this:
page1.php
<?php
session_start();
$_SESSION['myValue']=3; // You can set the value however you like.
?>
Any other PHP page:
<?php
session_start();
echo $_SESSION['myValue'];
?>
A few notes to keep in mind though: You need to call session_start() BEFORE any output, HTML, echos - even whitespace.
You can keep changing the value in the session - but it will only be able to be used after the first page - meaning if you set it in page 1, you will not be able to use it until you get to another page or refresh the page.
The setting of the variable itself can be done in one of a number of ways:
$_SESSION['myValue']=1;
$_SESSION['myValue']=$var;
$_SESSION['myValue']=$_GET['YourFormElement'];
And if you want to check if the variable is set before getting a potential error, use something like this:
if(!empty($_SESSION['myValue'])
{
echo $_SESSION['myValue'];
}
else
{
echo "Session not set yet.";
}