process_signup.php

<?php
mysqli_report(MYSQLI_REPORT_OFF);

if(empty($_POST["username"]))
{
  die("Username is required. Please go back to continue.");
}

if(!filter_var($_POST["email"], FILTER_VALIDATE_EMAIL))
{
  die("Valid email address is required. Please go back to continue.");
}

if(strlen($_POST["password"]) < 8)
{
  die("Password must be at least 8 characters long. Please go back to continue.");
}

// skipped lowercase and uppercase validation
// if(!preg_match("/[a-z]/i", $_POST["password"]))
// {
//   die("Password must contain at least one lowercase and at least one uppercase letter");
// }

if(!preg_match("/[0-9]/i", $_POST["password"]))
{
  die("Password must contain a number. Please go back to continue.");
}

if($_POST["password"] !== $_POST["confirm_password"])
{
  die("Password and confirm password must match. Please go back to continue.");
}

$password_hash = password_hash($_POST["password"], PASSWORD_DEFAULT);
$mysqli = require __DIR__ . "/database.php";
$sql = "INSERT INTO user (username, email, password_hash) VALUES (?, ?, ?)";
$stmt = $mysqli->stmt_init();

if(!$stmt->prepare($sql))
{
  die("SQL Error: " . $mysqli->error);
}

$stmt->bind_param("sss", $_POST["username"], $_POST["email"], $password_hash);

if($stmt->execute())
{
  header("Location: signup_success.php");
  exit;
}
else
{
  if($mysqli->errno === 1062)
  {
    die("Sorry, this email is already registered. Please LogIn to continue.");
  }
  else
  {
    die($mysqli->error . " " . $mysqli->errno);
  }
}

// print_r($_POST);
// var_dump($password_hash);
?>