Insert NULL values to MySql records using MySqli

My page sends using ajax data to a php script. one of the vars that the script gets is: $product_disc now a small test that I have done inside the script clearly revealed that $product_disc is null. the test:

if ($product_disc == null)
 {$test="null";}
 else {$test="not null";}

I make my code return $test to the ajax calling procedure:

$return_array['success'] = array ('test' => $test);

And using Firebug I see that the ajax response has: "test":"null"

So i conclude that $product_disc is of value: null

Now, inside my script I'm using the following code to insert data to MySql DB:

$stmt = mysqli_prepare($link, "INSERT INTO set_products (id,discount) VALUES (NULL , ?)");
mysqli_stmt_bind_param($stmt, "i", $product_disc);
mysqli_stmt_execute($stmt);
mysqli_stmt_close($stmt);

The query is being executed, but the value that was inserted is 0 and not NULL!

Only when I explicitly added: $product_disc=null; before the MySqli statements, the value that was inserted was NULL.

Why is that? What's the different between a null and a null? I followed this answer in stackoverflow hoping that i could easily insert NULLs but then came this problem...

Thank you!

PS @stackoverflow team - Your spell checking dict开发者_如何转开发ionary doesn't recognize the word stackoverflow!

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You prepare product_disc as integer, null is not an integer. It is converted to 0.

While preparing as a string, would definitely fix your issue, it defeats the purpose of preparing a mysql statement. Consider this solution:

mysqli_stmt_bind_param($stmt, $product_disc==null?'s':'i', $product_disc);