Hi,
Ok, thanks for the access, I see what you mean now.
So it's not really a PHP restriction you're talking about. It's just the limitation of the HTML input field.
You can easily change that on your end.
For that, go in the menu Display>Views and edit the file sub_block_login_registration.php for your frontend template.
There, you can find this line of HTML for the username input field:
<input type="text" name="data[register][username]" id="register_username" value="<?php echo $this->escape($this->mainUser->get( 'username' ));?>" class="hkform-control validate-username" maxlength="25" size="30" />
you can just increase the value of the "maxlength" attribute there to have more characters possible and save and you'll be good to go.