Contao\Widget::getEmptyValueByFieldType PHP Method

    public static function getEmptyValueByFieldType($sql)
    {
        if ($sql == '') {
            return '';
        }
        if (stripos($sql, 'NOT NULL') === false) {
            return null;
        }
        $type = strtolower(preg_replace('/^([A-Za-z]+)(\\(| ).*$/', '$1', $sql));
        if (in_array($type, array('int', 'integer', 'tinyint', 'smallint', 'mediumint', 'bigint', 'float', 'double', 'dec', 'decimal'))) {
            return 0;
        }
        return '';
    }

 /**
  * Save the current value
  *
  * @param mixed $varValue
  *
  * @throws \Exception
  */
 protected function save($varValue)
 {
     if (\Input::post('FORM_SUBMIT') != $this->strTable) {
         return;
     }
     $arrData = $GLOBALS['TL_DCA'][$this->strTable]['fields'][$this->strField];
     // Convert date formats into timestamps
     if ($varValue != '' && in_array($arrData['eval']['rgxp'], array('date', 'time', 'datim'))) {
         $objDate = new \Date($varValue, \Date::getFormatFromRgxp($arrData['eval']['rgxp']));
         $varValue = $objDate->tstamp;
     }
     // Make sure unique fields are unique
     if ($arrData['eval']['unique'] && $varValue != '' && !$this->Database->isUniqueValue($this->strTable, $this->strField, $varValue, $this->objActiveRecord->id)) {
         throw new \Exception(sprintf($GLOBALS['TL_LANG']['ERR']['unique'], $arrData['label'][0] ?: $this->strField));
     }
     // Handle multi-select fields in "override all" mode
     if (\Input::get('act') == 'overrideAll' && ($arrData['inputType'] == 'checkbox' || $arrData['inputType'] == 'checkboxWizard') && $arrData['eval']['multiple']) {
         if ($this->objActiveRecord !== null) {
             $new = deserialize($varValue, true);
             $old = deserialize($this->objActiveRecord->{$this->strField}, true);
             // Call load_callback
             if (is_array($GLOBALS['TL_DCA'][$this->strTable]['fields'][$this->strField]['load_callback'])) {
                 foreach ($GLOBALS['TL_DCA'][$this->strTable]['fields'][$this->strField]['load_callback'] as $callback) {
                     if (is_array($callback)) {
                         $this->import($callback[0]);
                         $old = $this->{$callback[0]}->{$callback[1]}($old, $this);
                     } elseif (is_callable($callback)) {
                         $old = $callback($old, $this);
                     }
                 }
             }
             switch (\Input::post($this->strInputName . '_update')) {
                 case 'add':
                     $varValue = array_values(array_unique(array_merge($old, $new)));
                     break;
                 case 'remove':
                     $varValue = array_values(array_diff($old, $new));
                     break;
                 case 'replace':
                     $varValue = $new;
                     break;
             }
             if (!is_array($varValue) || empty($varValue)) {
                 $varValue = \Widget::getEmptyStringOrNullByFieldType($arrData['sql']);
             } elseif (isset($arrData['eval']['csv'])) {
                 $varValue = implode($arrData['eval']['csv'], $varValue);
                 // see #2890
             } else {
                 $varValue = serialize($varValue);
             }
         }
     }
     // Convert arrays (see #2890)
     if ($arrData['eval']['multiple'] && isset($arrData['eval']['csv'])) {
         $varValue = implode($arrData['eval']['csv'], deserialize($varValue, true));
     }
     // Trigger the save_callback
     if (is_array($arrData['save_callback'])) {
         foreach ($arrData['save_callback'] as $callback) {
             if (is_array($callback)) {
                 $this->import($callback[0]);
                 $varValue = $this->{$callback[0]}->{$callback[1]}($varValue, $this);
             } elseif (is_callable($callback)) {
                 $varValue = $callback($varValue, $this);
             }
         }
     }
     // Save the value if there was no error
     if (($varValue != '' || !$arrData['eval']['doNotSaveEmpty']) && ($this->varValue !== $varValue || $arrData['eval']['alwaysSave'])) {
         // If the field is a fallback field, empty all other columns (see #6498)
         if ($arrData['eval']['fallback'] && $varValue != '') {
             if ($GLOBALS['TL_DCA'][$this->strTable]['list']['sorting']['mode'] == 4) {
                 $this->Database->prepare("UPDATE " . $this->strTable . " SET " . $this->strField . "='' WHERE pid=?")->execute($this->activeRecord->pid);
             } else {
                 $this->Database->execute("UPDATE " . $this->strTable . " SET " . $this->strField . "=''");
             }
         }
         // Set the correct empty value (see #6284, #6373)
         if ($varValue === '') {
             $varValue = \Widget::getEmptyValueByFieldType($GLOBALS['TL_DCA'][$this->strTable]['fields'][$this->strField]['sql']);
         }
         $arrValues = $this->values;
         array_unshift($arrValues, $varValue);
         $objUpdateStmt = $this->Database->prepare("UPDATE " . $this->strTable . " SET " . $this->strField . "=? WHERE " . implode(' AND ', $this->procedure))->execute($arrValues);
         if ($objUpdateStmt->affectedRows) {
             $this->blnCreateNewVersion = true;
             $this->varValue = deserialize($varValue);
             if (is_object($this->objActiveRecord)) {
                 $this->objActiveRecord->{$this->strField} = $this->varValue;
             }
         }
     }
 }