undefined index when page is loaded - php

Question

I'm having a issue where a variable is becoming undefined when the page is loaded individually...
So.
My front page has a address form where when the address is filled out and you click "Get your offer" it'll take you to another page where the address is carried over using $_POST['address'] in the value of the new input. so value="<?php echo $_POST['address']; ?>"
My problem is that when the offer page is loaded without using the front page form it gives me the error
<br /><b>Notice</b>: Undefined index: address in <b>C:\xampp\htdocs\offer\index.php</b> on line <b>228</b><br />Address
which makes sense. so i tried to fix it by putting this in the value= :
<?php
$carryover = $_POST['address'] or empty($carryover);
if(empty($carryover)) {
echo 'Enter Address';
} else {
echo $carryover;
}
?>
which did absolutely nothing so.
Front page form:
<form method="post" action="/offer/index.php" name="front" id="front">
<div class="form-group">
<input type="text" id="autocomplete" onFocus="geolocate()" name="address" id="address" value="" placeholder="123 main st" required>
<button type="submit" class="theme-btn btn-style-nine"><span class="txt">Get your offer</span></button>
</div>
</form>
Form 2 on offer page (where address is carried over too):
<form class="multisteps-form__form" action="finish.php" id="wizard" method="POST" enctype="multipart/form-data">
<div class="col-lg-8">
<div class="form-input-inner position-relative has-float-label" >
<input type="text" name="address" id="address" placeholder="Address" value="<?php
$carryover = $_POST['address'];
if(empty($carryover)) {
echo 'Address';
} else{
echo $carryover;
}
?>
" class="form-control" required>
<label>Address</label>
<div class="icon-bg text-center">
<i class="fas fa-home"></i>
</div>
</div>
</div>
</form

Answer 1

The undefined index notice happens when you read $_POST['address'] so any code you write after that isn't going to affect the notice.
Checking whether $carryover is empty is too late, you need to check if the $_POST index itself is empty.
Instead of:
$carryover = $_POST['address'];
if(empty($carryover)) {
echo 'Enter Address';
} else {
echo $carryover;
}
You need to use:
if(empty($_POST['address']) {
echo 'Enter Address';
} else {
$carryover = $_POST['address'];
echo $carryover;
}

Answer 2

Instead of this:
$carryover = $_POST['address'] or empty($carryover);
use this:
$carryover = $_POST['address'] ?? '';