too long

I have a forum and visitors can register user names with more than 20 letters, how can I deny those visitors to register with a (long) user names?

the registration form is using PHP language.

thanks,

You can add a maxlength attribute to your HTML form as a first-step measure.

<input type="text" name="username" maxlength="20" />

On the PHP side, you can use strlen to check the length of your passed parameter.

Make a condition.

if (strlen($_POST['whatever-username-field']) > 20){
   // not allowed to proceed 
}

Use strlen() function which is supposed to mean string length like this:

if(strlen($username) > 20) {echo "You can not enter more than 20 characters";}

Or in your html form use the max length attribute. like:

  Username: <input type="text" name="usrname" maxlength="20" />