Get the name of the user in yii2

$user_id = Yii::$app->user->id;
$user_name = User::find()->where(['id'=>$user_id])->one()->username;

While the answer from @thepeach works, you can actually extend the User component and add your own functions, so that you can get them via Yii::$app->user->something as you were initially trying to do.

I like to extend things like this from the start, so I am ready to add custom functionality without having to refactor any code. It sucks to do things one way, then have to go back and fix 100 spots of code, because you changed it later.

First, define a user component class in your config:

'components' => [
    'user' => [
        'class' => 'app\components\User', // extend User component
    ],
],

Then create User.php in your components directory. If you haven't made this directory, create it in your app root.

User.php

<?php
namespace app\components;

use Yii;

/**
 * Extended yii\web\User
 *
 * This allows us to do "Yii::$app->user->something" by adding getters
 * like "public function getSomething()"
 *
 * So we can use variables and functions directly in `Yii::$app->user`
 */
class User extends \yii\web\User
{
    public function getUsername()
    {
        return \Yii::$app->user->identity->username;
    }

    public function getName()
    {
        return \Yii::$app->user->identity->name;
    }
}

Now you can access these through Yii::$app->user->something.

For example, put this in one of your views and access the page in your browser:

<?= \Yii::$app->user->username ?>

---

I wrote a more detailed answer here, which covers this a bit more in depth.

Easy, just use:

<?= \Yii::$app->user->identity->username ?>

On login the user information will be stored in Yii::$app->user->identity variable.

For more information have a read through the User Authentication documentation in the official guide.