注册密码和用户名的最小字符

如何强制密码和用户名在PDO中具有最小长度?我可以使用PHP在一个简单的输入字段中自己完成，但是使用这个PDO注册系统，我不知道如何做到这一点，或者在哪里做到这一点。我不擅长PHP，更不擅长PDO。

注册表单:

<?php 
ob_start();
    // This if statement checks to determine whether the registration form has been submitted 
    // If it has, then the registration code is run, otherwise the form is displayed 
    if(!empty($_POST)) {
        // Ensure that the user has entered a non-empty username 
        if(empty($_POST['username'])) 
        { 
            // Note that die() is generally a terrible way of handling user errors 
            // like this.  It is much better to display the error with the form 
            // and allow the user to correct their mistake.  However, that is an 
            // exercise for you to implement yourself. ;
            die('
                        <div class="notice fail">
                    <div class="notice-p">
                        Something went wrong!<br />
                        Please enter a username
                    </div>
                </div><br />
                '); 
        } 
        // Ensure that the user has entered a non-empty password 
        if(empty($_POST['password'])) 
        { 
            die('
                <div class="notice fail">
                    <div class="notice-p">
                        Something went wrong!<br />
                        Please enter a password
                    </div>
                </div><br />
                '); 
        } 
        // Make sure the user entered a valid E-Mail address 
        // filter_var is a useful PHP function for validating form input, see: 
        // http://us.php.net/manual/en/function.filter-var.php 
        // http://us.php.net/manual/en/filter.filters.php 
        if(!filter_var($_POST['email'], FILTER_VALIDATE_EMAIL)) 
        { 
            die('
                <div class="notice fail">
                    <div class="notice-p">
                        Something went wrong!<br />
                        Invalid E-mail address.
                    </div>
                </div><br />
                '); 
        } 
        // We will use this SQL query to see whether the username entered by the 
        // user is already in use.  A SELECT query is used to retrieve data from the database. 
        // :username is a special token, we will substitute a real value in its place when 
        // we execute the query. 
        $query = " 
            SELECT 
                1 
            FROM users 
            WHERE 
                username = :username 
        "; 
        // This contains the definitions for any special tokens that we place in 
        // our SQL query.  In this case, we are defining a value for the token 
        // :username.  It is possible to insert $_POST['username'] directly into 
        // your $query string; however doing so is very insecure and opens your 
        // code up to SQL injection exploits.  Using tokens prevents this.
        // For more information on SQL injections, see Wikipedia: 
        // http://en.wikipedia.org/wiki/SQL_Injection 
        $query_params = array( 
            ':username' => $_POST['username'] 
        ); 
        try 
        { 
            // These two statements run the query against your database table. 
            $stmt = $db->prepare($query); 
            $result = $stmt->execute($query_params); 
        } 
        catch(PDOException $ex) 
        { 
            // Note: On a production website, you should not output $ex->getMessage(). 
            // It may provide an attacker with helpful information about your code. 
            die('
                <div class="notice fail">
                    <div class="notice-p">
                        Something went wrong!<br />
                        Please try agian.
                    </div>
                </div><br />
                ' . $ex->getMessage()); 
        } 
        // The fetch() method returns an array representing the "next" row from 
        // the selected results, or false if there are no more rows to fetch. 
        $row = $stmt->fetch(); 
        // If a row was returned, then we know a matching username was found in 
        // the database already and we should not allow the user to continue. 
        if($row) 
        { 
            die('
                <div class="notice fail">
                    <div class="notice-p">
                        Something went wrong!<br />
                        Username is already taken.
                    </div>
                </div><br />
                '); 
        } 
        // Now we perform the same type of check for the email address, in order 
        // to ensure that it is unique. 
        $query = " 
            SELECT 
                1 
            FROM users 
            WHERE 
                email = :email 
        "; 
        $query_params = array( 
            ':email' => $_POST['email'] 
        ); 
        try 
        { 
            $stmt = $db->prepare($query); 
            $result = $stmt->execute($query_params); 
        } 
        catch(PDOException $ex) 
        { 
            die('
                <div class="notice fail">
                    <div class="notice-p">
                        Something went wrong!<br />
                        Please try again.
                    </div>
                </div><br />
                ' . $ex->getMessage()); 
        } 
        $row = $stmt->fetch(); 
        if($row) 
        { 
            die('
                <div class="notice fail">
                    <div class="notice-p">
                        Something went wrong!<br />
                        This E-mail is already in use by someone ells.
                    </div>
                </div><br />
                '); 
        } 
        // An INSERT query is used to add new rows to a database table. 
        // Again, we are using special tokens (technically called parameters) to 
        // protect against SQL injection attacks. 
        $query = " 
            INSERT INTO users ( 
                username, 
                password, 
                salt, 
                email 
            ) VALUES ( 
                :username, 
                :password, 
                :salt, 
                :email 
            ) 
        "; 
        // A salt is randomly generated here to protect again brute force attacks 
        // and rainbow table attacks.  The following statement generates a hex 
        // representation of an 8 byte salt.  Representing this in hex provides 
        // no additional security, but makes it easier for humans to read.
        // For more information: 
        // http://en.wikipedia.org/wiki/Salt_%28cryptography%29 
        // http://en.wikipedia.org/wiki/Brute-force_attack 
        // http://en.wikipedia.org/wiki/Rainbow_table 
        $salt = dechex(mt_rand(0, 2147483647)) . dechex(mt_rand(0, 2147483647)); 
        // This hashes the password with the salt so that it can be stored securely 
        // in your database.  The output of this next statement is a 64 byte hex 
        // string representing the 32 byte sha256 hash of the password.  The original 
        // password cannot be recovered from the hash.  For more information: 
        // http://en.wikipedia.org/wiki/Cryptographic_hash_function 
        $password = hash('sha256', $_POST['password'] . $salt); 
        // Next we hash the hash value 65536 more times.  The purpose of this is to 
        // protect against brute force attacks.  Now an attacker must compute the hash 65537 
        // times for each guess they make against a password, whereas if the password 
        // were hashed only once the attacker would have been able to make 65537 different  
        // guesses in the same amount of time instead of only one. 
        for($round = 0; $round < 65536; $round++) 
        { 
            $password = hash('sha256', $password . $salt); 
        } 
        // Here we prepare our tokens for insertion into the SQL query.  We do not 
        // store the original password; only the hashed version of it.  We do store 
        // the salt (in its plaintext form; this is not a security risk). 
        $query_params = array( 
            ':username' => $_POST['username'], 
            ':password' => $password, 
            ':salt' => $salt, 
            ':email' => $_POST['email'] 
        ); 
        try 
        { 
            // Execute the query to create the user 
            $stmt = $db->prepare($query); 
            $result = $stmt->execute($query_params); 
        } 
        catch(PDOException $ex) 
        { 
            // Note: On a production website, you should not output $ex->getMessage(). 
            // It may provide an attacker with helpful information about your code.  
            die('
                      <div class="notice fail">
                    <div class="notice-p">
                        Something went wrong!<br />
                        Please try again.
                    </div>
                </div><br />
                ' . $ex->getMessage()); 
        } 
        ob_clean();
        // This redirects the user back to the login page after they register 
        header("Location: /signin/"); 
        // Calling die or exit after performing a redirect using the header function 
        // is critical.  The rest of your PHP script will continue to execute and 
        // will be sent to the user if you do not die or exit. 
        die(); 
    } 

//session to store input after die() function
?>