Passing a value IF set - php

Question

I am trying to set up an application form on my site where there is a main large application form on one page (parent.php)and a smaller form on another page (child2.php) that when users fill out some of their details and submit that smaller form, they are taken to the larger form and the details they have entered already appear in the corresponding textbox on the larger form along side some extra boxes for them to fill out(if that makes sense!)
I can get it to work in that the textboxes on the 2nd page display the values of the matching textboxes on the first page, but only when a value is set. As users can either access the main application form through the smaller form OR by directly accessing it, I need to have it so that if the value is set, the set value is displayed and will also be the value entered into the database OR if the value is not pre-set from the smaller form, the user can enter in their info to the main form and this is what's sent to the DB. I think I might need to use ifisset and have tried to do so but am getting nowhere.
Apologies for messy code and the set up of the textboxes as they are just for testing this out and I am still getting to grips with all this and would be grateful if anyone could help me/let me know if I'm on the right track/totally off. Thanks in advance!
Page 1 (parent.php)
<form action="child2.php" method="post" class="validate">
<div>
<input class="tb" type="text" name="fName" placeholder="first name" id="fName" value="<?php $fName ?>" required/><br/>
<br/>
<input class="tb" type="text" name="sName" placeholder="surname" id="sName" value="<?php $sName ?>" required/><br/>
<br/>
<input class="tb" type="email" name="email" required placeholder="email address" id="email" value="<?php $email ?>" required/>
<br/>
<input class="tb" type="address" name="address" placeholder="address" value="<?php $address ?>" id="address" />
<br/>
<input id="submit" name="submit" type="submit" value="Submit">
</div>
</form>
Page 2 (child2.php)
<?php
function renderForm($fName, $sName, $email, $address){
?>
<form action="" method="post" class="validate">
<label class="label">first name</label><input class="tb" type="text" id="fName" name="fName" value="<?php if (isset($fName)) { echo $fName = $_REQUEST['fName'];} else { echo "first name"; }?>"/>
</br>
<label class="label">surname</label><input class="tb" type="text" id="sName" name="sName" value="<?php if (isset($sName)) { echo $sName = $_REQUEST['sName'];} else { echo "surname"; }?>"/>
</br>
<label class="label">email</label><input class="tb" type="email" id="email" name="email" value="<?php if (isset($email)) { echo $email = $_REQUEST['email'];} else { echo "email"; }?>"/>
</br>
<label class="label">address</label><input class="tb" type="text" id="address" name="address" value="<?php if (isset($address)) { echo $address = $_REQUEST['address'];} else { echo "address"; }?>"/>
</br>

Answer 1

What you're looking for are $_SESSION variables, which stay active until the user closes the browser or until the Session expires (I think the default PHP config time is 24 minutes..?). This is a lot more efficient than storing them in a database for temporal purposes.
So you can set the variables with $_SESSION['fName'] = $_POST['fName']; etc. and then call them later with echo $_SESSION['fName']; etc.
To utilize session variables you will need <?php session_start(); ?> at the beginning of your pages (before any HTML is used)

Answer 2

As suggested by khanahk, using SESSION variables will do your job. The problem with the solution you are thinking is that its fine only as long as you are passing values from one page to some other page, that too after so much mess.
So, you should instead, you should use something like this
session_start();
$email = $_SESSION['email'];
<label class="label">email</label><input class="tb" type="email" id="email" name="email" value="<?php echo $email; ?>"/>