yii\validators\RequiredValidator::validateValue PHP Method

    protected function validateValue($value)
    {
        if ($this->requiredValue === null) {
            if ($this->strict && $value !== null || !$this->strict && !$this->isEmpty(is_string($value) ? trim($value) : $value)) {
                return null;
            }
        } elseif (!$this->strict && $value == $this->requiredValue || $this->strict && $value === $this->requiredValue) {
            return null;
        }
        if ($this->requiredValue === null) {
            return [$this->message, []];
        } else {
            return [$this->message, ['requiredValue' => $this->requiredValue]];
        }
    }