<?php
$old_array = array(
array("id" => 23 , "my_value"=>NULL),
array("id" => 65 , "my_value"=>45)
);
echo "Old array: ";
var_dump($old_array);
foreach($old_array as $value)
$new_array[$value['id']]= ($value['my_value']!==NULL) ? $value['my_value'] : 100 ;
echo "New array: ";
var_dump($new_array);
?>