I'm kinda new to PHP and I'm trying to make a script to validate a HTML form only with PHP (I know there are options to do so with js, but I guess it's a better thing to have a "cover" with a server-side script). So:

project/
    classes/ #contains classes to validate and process the HTML form
        DB.php #contains the connection to the MySQL server
        Rules.php #contains all rules to validate the form
        Validate.php #contains methods to validate each field
        Register.php #is the registration class
        ini.php #contains the spl_autoload_register() function
    index.php #contains the HTML form

HTML form code, which is located in the project folder

<?php include_once 'lib/csrf-magic/csrf-magic.php; ?>
<form method="POST" action="classes/Register.php">
<fieldset>
    <legend><strong>register:</strong></legend>
    * <input type="text" name="fname" placeholder="First Name"/><br/>
    <div style="float:left;">
        <?php 
        // I want to put here all the validation errors for the fname field, if any
        ?>
    </div>

    <input type="submit" name="submit" value="Register"/>
</fieldset>
</form>

classes/ini.php

error_reporting(E_ALL);
ini_set('display_errors','On');

spl_autoload_register(function($class){
    require_once $class.'.php';
});

classes/DB.php

class DB
{
    private $driver = 'mysql';
    private $host   = '127.0.0.1';
    private $user   = 'root';
    private $pass   = '';
    private $name   = 'project';
    public $db;

    public function __construct()
    {
        try
        {
            $this->db = new PDO("$this->driver:host=$this->host;dbname=$this->name;charset=utf8", $this->user, $this->pass);
        
            //Error reporting. Throw exceptions
            $this->db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

            //Use native prepared statements
            $this->db->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);

            //Don't use persistent connections across multiple server sessions
            $this->db->setAttribute(PDO::ATTR_PERSISTENT, false);

            //Set default fetch mode
            $this->db->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_OBJ);

            //Return the connection object
            return $this->db;
        } catch (PDOException $e)
        {
            #echo 'Sorry. We have some problemes. Try again later!';
            echo $e->getMessage();
        }
    }
}

classes/Rules.php

class Rules
{
    /**
     * used for displaying in the page the user input text
     */
    public static function escape($str)
    {
        return htmlspecialchars(trim($str),ENT_QUOTES,'UTF-8');
    }

    /**
      * used to allow only numbers, letters (lc+uc), a space, an underscore, and a dash for a username. this will be stored into db
      */
    public static function filter($str)
    {
        return preg_replace('#[^0-9a-z _-]#i', "", trim($str));
    }

    /**
      * used to determine if the input has a certain minimum length
      */
    public static function minlen($str, $value)
    {
        return mb_strlen(trim($str)) < $value ? true : false;
    }

    /**
      * used to determine if the input has a certain maximum length
      */
    public static function maxlen($str, $value)
    {
        return mb_strlen(trim($str)) > $value ? true : false;
    }

    /**
      * used to determine if the password has a certain minimum length
      */
    public static function passLen($str)
    {
        return mb_strlen(trim($str)) < 6 ? true : false;
    }

    /**
      * used to determine if two passwords are equal
      */
    public static function equals($pass1, $pass2)
    {
        return trim($pass2) === trim($pass1) ? true : false;
    }

    /**
      * used to determine if a textbox contain a valid email
      */
    public static function email($email)
    {
        return filter_var(trim($email),FILTER_VALIDATE_EMAIL) ? true : false;
    }

    /**
      * used to determine if a user has wrote some date into the textbox
      */
    public static function required($str)
    {
        return trim($str) === '' ? true : false;
    }
}

classes/Validation.php

require 'ini.php';

class Validation{
    public static function validateName($name){
        $errors['name'] = [];
        $name = Rules::filter($name);

        if (Rules::required($name)) {
            $errors['name'][] = 'The Name field is mandatory<br/>';
        }
        if (Rules::minlen($name, 3)) {
            $errors['name'][] = 'The Name field is too short<br/>';
        }
        if (Rules::maxlen($name, 50)) { // this field is 50 chars long into the db table
            $errors['name'][] = 'The Name field is too long<br/>';
        }
    
        if (isset($errors['name'])) {
            return $errors['name'];
        } else {
            return true;
        }
    }
}

classes/Register.php

require_once 'ini.php';

class Register
{
    private $db;

    public function __construct(DB $db)
    {
         $this->db = $db->db;
    }

    public function reg()
    {
        $fname = isset($_POST['fname']) ? $_POST['fname'] : ''; // I' use HTMLPurifier for this variable

        if (isset($_POST['submit'])) {
            if (Validation::validateName($fname)) {
                $stmt = $this->db->prepare('INSERT INTO users SET fname=:fn, lname=:ln');
                $stmt->bindValue(':fn', $fname, PDO::PARAM_STR);
                $stmt->exeecute();
            } else {
                foreach (Validation::validateName($fname) as $e) {
                    return $e;
                }
        }
    }
}

$d = new DB();
$r = new Register($d);
echo $r->reg();

Now, I want to put all the validation errors that will occur when the user submits the form w/o filling accordingly all the fields required, into the <div> located down under the <input type="text" name="fname"> field. The errors are in the foreach loop located into the Register class. Another major aspect is the security: what should I improve to my code? Thanks! Any suggestion, is kindly appreciated.

I'm kinda new to PHP and I'm trying to make a script to validate a HTML form only with PHP (I know there are options to do so with JS, but I guess it's better to have a "cover" with a server-side script).

project/
    classes/ #contains classes to validate and process the HTML form
        DB.php #contains the connection to the MySQL server
        Rules.php #contains all rules to validate the form
        Validate.php #contains methods to validate each field
        Register.php #is the registration class
        ini.php #contains the spl_autoload_register() function
    index.php #contains the HTML form

HTML form code, which is located in the project folder:

<?php include_once 'lib/csrf-magic/csrf-magic.php; ?>
<form method="POST" action="classes/Register.php">
<fieldset>
    <legend><strong>register:</strong></legend>
    * <input type="text" name="fname" placeholder="First Name"/><br/>
    <div style="float:left;">
        <?php 
        // I want to put here all the validation errors for the fname field, if any
        ?>
    </div>

    <input type="submit" name="submit" value="Register"/>
</fieldset>
</form>

classes/ini.php

error_reporting(E_ALL);
ini_set('display_errors','On');

spl_autoload_register(function($class){
    require_once $class.'.php';
});

classes/DB.php

class DB
{
    private $driver = 'mysql';
    private $host   = '127.0.0.1';
    private $user   = 'root';
    private $pass   = '';
    private $name   = 'project';
    public $db;

    public function __construct()
    {
        try
        {
            $this->db = new PDO("$this->driver:host=$this->host;dbname=$this->name;charset=utf8", $this->user, $this->pass);
        
            //Error reporting. Throw exceptions
            $this->db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

            //Use native prepared statements
            $this->db->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);

            //Don't use persistent connections across multiple server sessions
            $this->db->setAttribute(PDO::ATTR_PERSISTENT, false);

            //Set default fetch mode
            $this->db->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_OBJ);

            //Return the connection object
            return $this->db;
        } catch (PDOException $e)
        {
            #echo 'Sorry. We have some problemes. Try again later!';
            echo $e->getMessage();
        }
    }
}

classes/Rules.php

class Rules
{
    /**
     * used for displaying in the page the user input text
     */
    public static function escape($str)
    {
        return htmlspecialchars(trim($str),ENT_QUOTES,'UTF-8');
    }

    /**
      * used to allow only numbers, letters (lc+uc), a space, an underscore, and a dash for a username. this will be stored into db
      */
    public static function filter($str)
    {
        return preg_replace('#[^0-9a-z _-]#i', "", trim($str));
    }

    /**
      * used to determine if the input has a certain minimum length
      */
    public static function minlen($str, $value)
    {
        return mb_strlen(trim($str)) < $value ? true : false;
    }

    /**
      * used to determine if the input has a certain maximum length
      */
    public static function maxlen($str, $value)
    {
        return mb_strlen(trim($str)) > $value ? true : false;
    }

    /**
      * used to determine if the password has a certain minimum length
      */
    public static function passLen($str)
    {
        return mb_strlen(trim($str)) < 6 ? true : false;
    }

    /**
      * used to determine if two passwords are equal
      */
    public static function equals($pass1, $pass2)
    {
        return trim($pass2) === trim($pass1) ? true : false;
    }

    /**
      * used to determine if a textbox contain a valid email
      */
    public static function email($email)
    {
        return filter_var(trim($email),FILTER_VALIDATE_EMAIL) ? true : false;
    }

    /**
      * used to determine if a user has wrote some date into the textbox
      */
    public static function required($str)
    {
        return trim($str) === '' ? true : false;
    }
}

classes/Validation.php

require 'ini.php';

class Validation{
    public static function validateName($name){
        $errors['name'] = [];
        $name = Rules::filter($name);

        if (Rules::required($name)) {
            $errors['name'][] = 'The Name field is mandatory<br/>';
        }
        if (Rules::minlen($name, 3)) {
            $errors['name'][] = 'The Name field is too short<br/>';
        }
        if (Rules::maxlen($name, 50)) { // this field is 50 chars long into the db table
            $errors['name'][] = 'The Name field is too long<br/>';
        }
    
        if (isset($errors['name'])) {
            return $errors['name'];
        } else {
            return true;
        }
    }
}

classes/Register.php

require_once 'ini.php';

class Register
{
    private $db;

    public function __construct(DB $db)
    {
         $this->db = $db->db;
    }

    public function reg()
    {
        $fname = isset($_POST['fname']) ? $_POST['fname'] : ''; // I' use HTMLPurifier for this variable

        if (isset($_POST['submit'])) {
            if (Validation::validateName($fname)) {
                $stmt = $this->db->prepare('INSERT INTO users SET fname=:fn, lname=:ln');
                $stmt->bindValue(':fn', $fname, PDO::PARAM_STR);
                $stmt->exeecute();
            } else {
                foreach (Validation::validateName($fname) as $e) {
                    return $e;
                }
        }
    }
}

$d = new DB();
$r = new Register($d);
echo $r->reg();

Now, I want to put all the validation errors that will occur when the user submits the form w/o filling accordingly all the fields required, into the <div> located down under the <input type="text" name="fname"> field. The errors are in the foreach loop located into the Register class. Another major aspect is the security: how should I improve my code?