Blank variables and INT columns?

I have a column in a table which is has the data type of an integer. What I had in mind was that values added into that column will be either 0 - N, or just blank as in an empty variable (see below), but I'm not sure that this is possible?

if($resource) {
   $resource = $id - 2;
} else {
   $resource = "";
}

$result = mysql_query("INSERT INTO table (...,resource,...) VALUE开发者_JAVA技巧S (...,'$resource',...)");

If not, could I instead use the data type of VARCHAR, and then say:

if($resource) {
   $resource = $id - 2;
} else {
   $resource = "INVALID";
}

In that case, is there any conversion functions I'd have to do when extracting values from the column resource, or would numbers automatically be treated as integers?

If the field should be "white" for any reason, I think you should mark it as NULLABLE and use word NULL (not INVALID)

If a column is a number, never use a varchar, you will loose a lot of things (also, an int is smaller than a varchar)

EDIT 1: Code snippet to allow null values on column:

ALTER TABLE mytable MODIFY mycolumn INT;

If you specify it as:

ALTER TABLE mytable MODIFY mycolumn INT NOT NULL;

It will be not nullable, so it should be nullable by default if you didn't declare it differently

EDIT 2: Important note, the column must not be UNIQUE otherwise the value will be not nullable!