Store values from multiple form in $_POST - php

Question

I am trying to store values from multiple forms in the $_POST variable. The forms are on different pages and from what I can understand from the test I did, when I submit values from a form, they "overwrite" the values that where already in $_POST.
Here is an example of what I am trying to do :
Page 1 - form 1
<?php
session_start();
?>
<form method="post" action="page2.php">
First name : <input type="text" name="firstName" required ><br/><br/>
Last name : <input type="text" name="lastName" required ><br/><br/>
<input type="submit">
</form>
Page 2 - form 2
<?php
session_start();
?>
<form method="post" action="page3.php">
Age : <input type="text" name="age" required ><br/><br/>
City : <input type="text" name="city" required ><br/><br/>
<input type="submit">
</form>
Page 3 - results
<?php
session_start();
echo $_POST['firstName'].'<br/>';
echo $_POST['lastName'].'<br/>';
echo $_POST['age'].'<br/>';
echo $_POST['city'].'<br/>';
?>
The last page shows me only 'age' and 'city'. The values from the first form on page 1 are now undefined. Here is the an example of the result I get :
Notice: Undefined index: firstName on line 4
Notice: Undefined index: lastName on line 5
65
New York

Answer 1

in page2.php
put this
<input type="hidden" name="firstname" value="<?=$_POST['firstname']?>">
<input type="hidden" name="lastname" value="<?=$_POST['lastname']?>">

Answer 2

You can also use session variables like this:
//Page 1
<?php
session_start();
$_SESSION['user'] = array();
$_SESSION['user']['firstName'] = $_POST['firstName'];
$_SESSION['user']['lastName'] = $_POST['lastName'];
?>
//Page 2
<?php
session_start();
$_SESSION['user']['age'] = $_POST['age'];
$_SESSION['user']['city'] = $_POST['city'];
?>
//Result
<?php
session_start();
echo $_SESSION['user']['firstName'];
echo $_SESSION['user']['lastName'];
echo $_SESSION['user']['city'];
echo $_SESSION['user']['age'];
?>

Answer 3

As I can see in the code above, you are trying to post the data to two different files.
<form method="post" action="page2.php">
This has action page as page2.php.
<form method="post" action="page3.php">
Here you are submitting data to page3.php
And you are trying to access
echo $_POST['firstName'].'<br/>';
echo $_POST['lastName'].'<br/>';
in page3.php which you have passed to page2.php. Definitely this won't work as you can access the values submitted through a form in the action page of the form only.