PHP - What Is The Purpose Of Refactoring Software?

I was working on this project earlier on in the year, I have not posted here much.  I would like to get it over with and start something else. I have other files which look like the excerpt of code from a file below. I personally think that my code lacks structure and could be organized in a better fashion. I lost most of my progress and am having to backtrack and restore code. Is there a way to re-write this code and make it more readable? Please and thank you!

<?php
include('header.php');
require_once('dbcon/dbcon.php');
//include('functions.php');

if ($_SERVER['REQUEST_METHOD'] == 'POST') {
	// sanitize values before entering them into db, no bad seeds.
	$username = mysqli_real_escape_string($conn, $_POST['username']);
	$password = mysqli_real_escape_string($conn, $_POST['password']);
	$bio = mysqli_real_escape_string($conn, $_POST['bio']);
	$hashed_password = mysqli_real_escape_string($conn, password_hash($password, PASSWORD_DEFAULT));
	$email = mysqli_real_escape_string($conn, $_POST['email_address']);
	$confirmation_status = 0;

/*	function sanitizeValues($x, string $postString) {
		$x = mysqli_real_escape_string($conn, $_POST[$postString]);
	}*/

	$username_query = "SELECT * from profiles001 WHERE username='$username'";
	$result = mysqli_query($conn, $username_query);

	// if username exists do not continue...
	if (mysqli_num_rows($result) > 0) {
		header('Location: /soapbox/signup.php');
		// let user know that username is taken...
	} else {
		// file upload stuff...
		$file = $_FILES['file'];
		$fileName = $_FILES['file']['name'];
		$fileTmpName = $_FILES['file']['tmp_name'];
		$fileSize = $_FILES['file']['size'];
		$fileError = $_FILES['file']['error'];
		$fileType = $_FILES['file']['type'];
		$fileExt = explode('.', $fileName);
		$fileActualExt = strtolower(end($fileExt));

		$allowed = array('jpg', 'jpeg', 'png');

		// avatar file constraints checks...
		if (in_array($fileActualExt, $allowed)) {
			if ($fileError === 0) {
				if ($fileSize < 1000000) {
					$fileNameNew = uniqid($_SESSION['username'], true) . "." . $fileActualExt;
					$fileDestination = 'uploads/' . $fileNameNew;
					move_uploaded_file($fileTmpName, $fileDestination);

				} else {
					echo "Your file is too big!";
				}
			} else {
				echo "There was an error uploading your file" . $fileError . $fileSize;
			}
		} else if (!(empty(in_array($fileActualExt, $allowed))) && !($allowed)) {
			echo "Cannot upload file of this type!";
		}

		mkdir("channel/" . $username);
		mkdir("channel/" . $username . "/videos");
		fopen("channel/" . $username . "/index.php", "w");

		$account_open_date = date("Y-m-d h:i:s");
		$current_date = date("Y-m-d h:i:s");
		//$account_open_date_retrieval_sql_select = "SELECT account_open_date from profile0";
		//$account_age = date_diff($row, $current_date); // acct open date - current date = account age
		//$account_age_result = mysqli_query($conn, $account_open_date_retrieval_sql_select);
		//$row = mysqli_fetch_assoc($account_age_result);

		// if-then-else-if statement to get rid of the fileDestination var undefined error when avatar photo is not submitted....
		if (!(empty($fileDestination))) {
			$sqlinsert = "INSERT INTO profiles001 (username, password, email, c_status, doc, avatar, bio) VALUES ('$username', '$hashed_password', '$email', '$confirmation_status', '$account_open_date', '$fileDestination', '$bio')";
		} else if (empty($fileDestination)) {
			$fileDestination = "assets/soap.jpg";
			$sqlinsert = "INSERT INTO profiles001 (username, password, email, c_status, doc, avatar, bio) VALUES ('$username', '$hashed_password', '$email', '$confirmation_status', '$account_open_date', '$fileDestination', '$bio')";
		}

		$result = mysqli_query($conn, $sqlinsert);
	}
}
?>

class AddPdfPage
{
    function addA(&$pdf, $vars) {}
    function addB(&$pdf, $vars) {}
    function addC(&$pdf, $vars) {}
    function addD(&$pdf, $vars) {}
}

//The way I call it from within the code right now is like this:
AddPdfPage::addC($pdf, $vars);
AddPdfPage::addB($pdf, $vars);


//the variable below contains the product line -- A, B, C, or D
//that's the one I said I could use to merge functions B/C/D into one if I wanted to
//I could even include product A if I wanted to even though the code there is different.
$vars['productLine']; 

//The code inside addX() functions is something like:
function addX($pdf, $vars)
{
    $pdf->addPage($vars);
    $pdf->addDescription(".."); //that is different based on each specific product line
}

class AddPdfPage()
{
    private $pdf;
    private $vars;
    private $productLine;

    function __construct($pdf, $vars)
    {
        $this->pdf = $pdf;
        $this->vars = $vars;
        $this->productLine = $vars['productLine'];
    }

    public function addPage()
    {
        switch($this->productLine)
        {
             case 'A': ...break;
             case 'B': ...break;
             case 'C': ...break;
             case 'D': ...break;
        }
    }
}