MySQL not inserting NULL Value - php

Question

This is going to be an easy one for you but I'm a noob so I need help.
Here is my code, for whatever reason the else statement isn't executing. If I change the NULL value for a $variable = NULL it works, but enters an empty string into my database rather than the NULL value. Can anyone help with why?
if (isset($_POST['agreeto']))
{
$enter_feedback = "INSERT INTO feedback" . //(initial, surname, country_id, date, friend_score, return_score, service_score, comments)
" VALUES('" . $initial. "', '" . $lastname . "', '" . $country_id . "', NOW(), '" . $friend . "', '" . $return . "', '" . $service . "', '" . $_POST['comments'] . "')";
}else
{
$enter_feedback = "INSERT INTO feedback" . //(initial, surname, country_id, date, friend_score, return_score, service_score, comments)
" VALUES('" . $initial. "', '" . $lastname . "', '" . $country_id . "', NOW(), '" . $friend . "', '" . $return . "', '" . $service . "', " . NULL . ")" or die("couldn't execute my query");
}

Answer 1

MySQL requires the string=NULL and you are passing it the value of the php NULL constant.
if (isset($_POST['agreeto']))
{
$enter_feedback = "INSERT INTO feedback" . //(initial, surname, country_id, date, friend_score, return_score, service_score, comments)
" VALUES('" . $initial. "', '" . $lastname . "', '" . $country_id . "', NOW(), '" . $friend . "', '" . $return . "', '" . $service . "', '" . $_POST['comments'] . "')";
}else
{
$enter_feedback = "INSERT INTO feedback" . //(initial, surname, country_id, date, friend_score, return_score, service_score, comments)
" VALUES('" . $initial. "', '" . $lastname . "', '" . $country_id . "', NOW(), '" . $friend . "', '" . $return . "', '" . $service . "',NULL)" or die("couldn't execute my query");
}

Answer 2

This should work as expected.
Try set the column to int, if this does not work.
INSERT INTO table (val1, val2) VALUES('String Value', NULL);
or set default column value to NULL.

Answer 3

A NULL value can not be enclosed in 'apostrophes'
Think about how you will get the SQL:
INSERT INTO table_name VALUES ('null','');?
The database may understand as a string

Answer 4

Just replace the 'NULL' value with NULL without the quotes.
$enter_feedback = "INSERT INTO feedback" . //(initial, surname, country_id, date, friend_score, return_score, service_score, comments)
" VALUES('" . $initial. "', '" . $lastname . "', '" . $country_id . "', NOW(), '" . $friend . "', '" . $return . "', '" . $service . "', NULL) " or die("couldn't execute my query");
Once you do that and save the record you will see NULL in the database column for that field when you browse via PHPMYADMIN. Please make sure that the field in question "comments" allows NULL.