Yii2: ActiveForm field numeric, length => 8

Try with integer data type:

[['username'], 'integer'],
[['username'], 'string', 'min' => 8],

It will validate both numeric and length. This will do the trick.

Read more about the validations here: http://www.yiiframework.com/doc-2.0/guide-tutorial-core-validators.html#number

This works fine for me using the contact form from yii2-basic

/**
 * @return array the validation rules.
 */
public function rules()
{
    return [
        // name, email, subject and body are required
        [['name', 'email', 'subject', 'body'], 'required'],
        // email has to be a valid email address
        ['email', 'email'],
        ['subject', 'is8NumbersOnly'],
        // verifyCode needs to be entered correctly
        ['verifyCode', 'captcha'],
    ];
}

public function is8NumbersOnly($attribute)
{
    if (!preg_match('/^[0-9]{8}$/', $this->$attribute)) {
        $this->addError($attribute, 'must contain exactly 8 digits.');
    }
}