src/View/Helper/FormButtonIcon.php
public function openTag($attributesOrElement = null)
    {
        if (null === $attributesOrElement) {
            return '<button>';
        }

        if (is_array($attributesOrElement)) {
            $attributes = $this->createAttributesString($attributesOrElement);

            return sprintf('<button %s>', $attributes);
        }

        if (!$attributesOrElement instanceof ElementInterface) {
            throw new Exception\InvalidArgumentException(
                sprintf(
                    '%s expects an array or Zend\Form\ElementInterface instance; received "%s"',
                    __METHOD__,
                    (is_object($attributesOrElement) ? get_class($attributesOrElement) :
                        gettype($attributesOrElement))
                )
            );
        }

        $element    = $attributesOrElement;
        $attributes = $element->getAttributes();
        $name       = $element->getName();

        if ($name) {
            $attributes['name'] = $name;
        }

        $attributes['type'] = $this->getType($element);
        $classList          = [
            'btn',
            'btn-white'
        ];

        /** @noinspection UnSafeIsSetOverArrayInspection */
        if (isset($attributes['class'])) {
            $attributes['class'] = implode(
                ' ', array_unique(array_merge(explode(' ', $attributes['class']), $classList))
            );
        } else {
            $attributes['class'] = implode(' ', $classList);
        }

        return sprintf(
            '<button %s>',
            $this->createAttributesString($attributes)
        );
    }
public function openTag($attributesOrElement = null)
    {
        if (null === $attributesOrElement) {
            return '<button>';
        }

        if (is_array($attributesOrElement)) {
            $attributes = $this->createAttributesString($attributesOrElement);

            return sprintf('<button %s>', $attributes);
        }

        if (!$attributesOrElement instanceof ElementInterface) {
            throw new Exception\InvalidArgumentException(
                sprintf(
                    '%s expects an array or Zend\Form\ElementInterface instance; received "%s"',
                    __METHOD__,
                    (is_object($attributesOrElement) ? get_class($attributesOrElement) :
                        gettype($attributesOrElement))
                )
            );
        }

        $element    = $attributesOrElement;
        $attributes = $element->getAttributes();
        $name       = $element->getName();

        if ($name) {
            $attributes['name'] = $name;
        }

        $attributes['type'] = $this->getType($element);
        $classList          = [
            'btn',
            'btn-white'
        ];

        /** @noinspection UnSafeIsSetOverArrayInspection */
        if (isset($attributes['class'])) {
            $attributes['class'] = implode(
                ' ', array_unique(array_merge(explode(' ', $attributes['class']), $classList))
            );
        } else {
            $attributes['class'] = implode(' ', $classList);
        }

        return sprintf(
            '<button %s>',
            $this->createAttributesString($attributes)
        );
    }
Generate an opening button tag.

@param array|ElementInterface $attributesOrElement

@return string
@throws \Zend\Form\Exception\InvalidArgumentException