framework/helpers/BaseHtml.php
public static function getAttributeValue($model, $attribute)
    {
        if (!preg_match(static::$attributeRegex, $attribute, $matches)) {
            throw new InvalidArgumentException('Attribute name must contain word characters only.');
        }
        $attribute = $matches[2];
        $value = $model->$attribute;
        if ($matches[3] !== '') {
            foreach (explode('][', trim($matches[3], '[]')) as $id) {
                if ((is_array($value) || $value instanceof \ArrayAccess) && isset($value[$id])) {
                    $value = $value[$id];
                } else {
                    return null;
                }
            }
        }

        // https://github.com/yiisoft/yii2/issues/1457
        if (is_array($value)) {
            foreach ($value as $i => $v) {
                if ($v instanceof ActiveRecordInterface) {
                    $v = $v->getPrimaryKey(false);
                    $value[$i] = is_array($v) ? json_encode($v) : $v;
                }
            }
        } elseif ($value instanceof ActiveRecordInterface) {
            $value = $value->getPrimaryKey(false);

            return is_array($value) ? json_encode($value) : $value;
        }

        return $value;
    }
public static function getAttributeValue($model, $attribute)
    {
        if (!preg_match(static::$attributeRegex, $attribute, $matches)) {
            throw new InvalidArgumentException('Attribute name must contain word characters only.');
        }
        $attribute = $matches[2];
        $value = $model->$attribute;
        if ($matches[3] !== '') {
            foreach (explode('][', trim($matches[3], '[]')) as $id) {
                if ((is_array($value) || $value instanceof \ArrayAccess) && isset($value[$id])) {
                    $value = $value[$id];
                } else {
                    return null;
                }
            }
        }

        // https://github.com/yiisoft/yii2/issues/1457
        if (is_array($value)) {
            foreach ($value as $i => $v) {
                if ($v instanceof ActiveRecordInterface) {
                    $v = $v->getPrimaryKey(false);
                    $value[$i] = is_array($v) ? json_encode($v) : $v;
                }
            }
        } elseif ($value instanceof ActiveRecordInterface) {
            $value = $value->getPrimaryKey(false);

            return is_array($value) ? json_encode($value) : $value;
        }

        return $value;
    }
Returns the value of the specified attribute name or expression.

For an attribute expression like `[0]dates[0]`, this method will return the value of `$model->dates[0]`.
See [[getAttributeName()]] for more details about attribute expression.

If an attribute value is an instance of [[ActiveRecordInterface]] or an array of such instances,
the primary value(s) of the AR instance(s) will be returned instead.

@param Model $model the model object
@param string $attribute the attribute name or expression
@return string|array the corresponding attribute value
@throws InvalidArgumentException if the attribute name contains non-word characters.