Magento 1.9 – System Config Multiselect Default All Selected

After some tests I can confirm this is a bug introduced by Magento 1.9.3.0 (see here: https://github.com/OpenMage/magento-mirror/commit/d48bebc211cc216aaf78bdf25d7f0b0143d6333b#diff-139e884940505308d9c796f5e3a78865 )

Side note: this also affects SUPEE-8788

As a temporary fix, here is what I suggest: copy app/code/core/Mage/Adminhtml/Model/System/Config/Backend/Serialized.php to app/code/local/Mage/Adminhtml/Model/System/Config/Backend/Serialized.php and modify the _afterLoad() method like this:

protected function _afterLoad()
{
    if (!is_array($this->getValue())) {
        $serializedValue = $this->getValue();
        $unserializedValue = false;
        if (!empty($serializedValue)) {
            try {
                $unserializedValue = Mage::helper('core/unserializeArray')
                    ->unserialize((string)$serializedValue);
            } catch (Exception $e) {
                Mage::logException($e);
            }
        }
        $this->setValue($unserializedValue);
    }
}

By default you can't. But you need some code for getting this.

Change Foolist class following way:

namespace Vendor\Module\Model\Config\Source;

use Magento\Framework\Option\ArrayInterface;

class Foolist implements ArrayInterface
{
    public $arr = array (
        1 => "foo",
        2 => "bar",
        4 => "toto",
        6 => "Bla"
    );

    public function toOptionArray()
    {
        $ret = [];
        foreach ($this->arr as $key => $value) {
            $ret[] = [
                'value' => $key,
                'label' => $value
            ];
        }

        return $ret;
    }

    public function getOriginalOption()
    {
        return $this->arr;
    }
}

Now you can use following code for getting your output result:

public function __construct(
    \Vendor\Module\Model\Config\Source\Foolist $foolist
) {
    $this->foolist = $foolist;
}

and

$entries = $this->_scopeConfig->getValue('section/group/foolist', \Magento\Store\Model\ScopeInterface::SCOPE_STORE);
$entries = explode(',', $entries);
$foolist = $this->foolist->getOriginalOption();
$result = array();
foreach($foolist as $key => $value) {
    if(in_array($key, $entries)) {
        $result[] = $value;
    }
}

print_r($result);