How to defeat NULL when compiling a query in the database?

Question

PHP

$valueString = join ( "','", $itemValues );
replaces null with ""

code next

$valueString = join ( "','", $itemValues );

      $valueString = "('" . $valueString . "'),";

      $values = "\n" . $valueString;

      if ($values != "") {
        $data_string = "INSERT INTO `$tableName` (`$items`) VALUES" . rtrim ( $values, "," ) . ";;;" . PHP_EOL;
        if ($this->fp)
          fwrite ( $this->fp, $data_string );
      }

accordingly I have
INSERT INTO `auth_item` (`name`,`type`,`description`,`rule_name`,`data`,`created_at`,`updated_at`) VALUES
('activity_stream_delete_comments','2','Delete comments' ,'','','1477591890','1477591890');;;
And in the database "" instead of null
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Answer 1

Accordingly, figure out how to correctly represent NULL in values ​​with NULL. This use of the join function will obviously have to be abandoned.
And it is better not to be perverted, and use pdo prepared statements, which null can be passed regularly.

Answer 2

'' !== NULL

Answer 3

$valueString = '';

foreach($itemValues as $val) {
$valueString = is_null($val) ? 'null,' : sprintf('"%s",', $val);
}
$valueString = rtrim($valueString, ',');