I want to create my own custom PHP,MySQL forum

For a practical, learning-first start: this builds on the suggestions from @WillGresham, @ShantiC and @martin5211 by giving a compact, hands‑on base you can extend. Below is a minimal schema (users/forums/threads/posts), a tiny PDO-based register/login example, and a short checklist of concrete things to implement or watch for as you grow the project.

CREATE TABLE users (
  id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  username VARCHAR(50) NOT NULL UNIQUE,
  email VARCHAR(255) NOT NULL UNIQUE,
  password_hash VARCHAR(255) NOT NULL,
  role ENUM('user','mod','admin') NOT NULL DEFAULT 'user',
  created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE forums (
  id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  title VARCHAR(150) NOT NULL,
  parent_id INT UNSIGNED DEFAULT NULL,
  position INT DEFAULT 0
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE threads (
  id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  forum_id INT UNSIGNED NOT NULL,
  user_id INT UNSIGNED NOT NULL,
  title VARCHAR(255) NOT NULL,
  slug VARCHAR(255) NOT NULL,
  created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  INDEX (forum_id), INDEX (user_id), INDEX (slug)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE posts (
  id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  thread_id INT UNSIGNED NOT NULL,
  user_id INT UNSIGNED NOT NULL,
  content MEDIUMTEXT NOT NULL,
  created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  is_deleted TINYINT(1) NOT NULL DEFAULT 0,
  INDEX (thread_id), INDEX (user_id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

// minimal examples (assumes $pdo is a configured PDO)
function register($pdo,$u,$e,$p){
  $hash = password_hash($p, PASSWORD_DEFAULT);
  $stmt = $pdo->prepare("INSERT INTO users (username,email,password_hash) VALUES (?,?,?)");
  return $stmt->execute([$u,$e,$hash]);
}
function login($pdo,$email,$password){
  $stmt = $pdo->prepare("SELECT id,password_hash FROM users WHERE email = ?");
  $stmt->execute([$email]); $user = $stmt->fetch(PDO::FETCH_ASSOC);
  if($user && password_verify($password,$user['password_hash'])){
    session_regenerate_id(true);
    $_SESSION['user_id']=$user['id'];
    return true;
  }
  return false;
}

Practical checklist and cautions:

• Use prepared statements everywhere; never concatenate user input into SQL.
• Store only hashed passwords (password_hash/password_verify). Enforce HTTPS and use secure, HttpOnly cookies.
• Design role/permission data so moderators and admins are explicit (permission table or bitmask). Add audit logs for moderator actions.
• Keep raw post text (Markdown or plain text), sanitize on render, and limit/scan uploads.
• Index foreign keys and paginate (keyset pagination for large forums). Consider external search later.
• Build a small plugin/event system and schema migrations from day one.

This gives a small, safe base to start from; harden incrementally and add tests for auth, permissions and input handling as you go.