我已经解决了将其添加到ActiveForm :: begin

I have solved adding this to ActiveForm::begin

<?php $form = ActiveForm::begin([

        'fieldConfig' => [
            'errorOptions' => ['class' => 'help-block', 'encode' => false],
    ],

]); ?>

和一个简单的<br />

 [['username'], 'required', 'message' => 'long message first line here <br />long message last line here'],