eZ\Publish\Core\FieldType\Selection\Type::validate PHP Method

    public function validate(FieldDefinition $fieldDefinition, SPIValue $fieldValue)
    {
        $validationErrors = array();
        if ($this->isEmptyValue($fieldValue)) {
            return $validationErrors;
        }
        $fieldSettings = $fieldDefinition->getFieldSettings();
        if ((!isset($fieldSettings['isMultiple']) || $fieldSettings['isMultiple'] === false) && count($fieldValue->selection) > 1) {
            $validationErrors[] = new ValidationError('Field definition does not allow multiple options to be selected.', null, array(), 'selection');
        }
        foreach ($fieldValue->selection as $optionIndex) {
            if (!isset($fieldSettings['options'][$optionIndex])) {
                $validationErrors[] = new ValidationError('Option with index %index% does not exist in the field definition.', null, array('%index%' => $optionIndex), 'selection');
            }
        }
        return $validationErrors;
    }