Q:

How do you properly set a blank variable as NULL

I am parsing an XML feed into a MYSQL table using simplexml_load_file().   Some of the variables are blank, I would like them to be actual NULL instead of NULL, or is there even a difference? 
$xml = simplexml_load_file('cb.xml') or die ('Bad XML File');
foreach($xml->item as $item) {
    $name = $item->name;

//Tried

if ($name == '') {
    $name = 'NULL';
}

//And

if ($name == '') {
    $name = NULL;
}

mysql_query("INSERT INTO cb (name) VALUES ('$name')");

A:

This is because you're giving MySQL a string:
.... ('ANYTHING WITHIN QUOTES IS A STRING')

And the PHP null value, when "casted" to a string, becomes an empty string.  So your first try gave ... ('NULL'), and now it gives ... (''). 
You must use the NULL keyword inside the query, without quotes, to insert NULL into a database field. 
mysql_query("INSERT INTO cb (name) VALUES (" . ( $name == null ? " NULL" : "'$name'") . ")"); 

Oh, and as usual, take care not to get SQL-injected with your unprotected $name variable.