trying to INSERT NULL if input field left blank - php

Question

I'm trying to insert NULL into the mySQL databse with PHP.
I've got:
$myVariable=$_POST[inputfield];
if(!is_numeric($myVariable) || $myVariable==0){
$myVariable=NULL;
}
My database field is decimal, and IS NULL has been set. It keeps reverting to 0.00 if input is left blank. How do I set NULL if left blank, so I can call a php if NULL function?
Edit Calling if NULL in decimal is not correct?
<? if ($debit==NULL || $debit=="0.00") {echo 'Awaiting Cost';}
elseif ($debit != NULL || $debit!="0.00") { echo "£$debit";}?>
This displays my message correctly, so if if NULL is useless for a decimal i'll just leave 0.00. Is this hacky?
SQL structure for those who asked:
`myTableField` decimal(10,2) default NULL,

Answer 1

If you are composing an SQL statement by appending the value of the $myVariable variable, then you should look if it's NULL or not and modify the SQL statement accordingly.
For example, if your code is something like:
$sql .= "myVariable = '" . mysql_real_escape_string($myVariable) . "'";
then you should change it to something like:
if (is_null($myVariable)) {
$sql .= "myVariable = NULL";
} else {
$sql .= "myVariable = '" . mysql_real_escape_string($myVariable) . "'";
}

Answer 2

Try it on the SQL side:
$sql = "INSERT INTO `table` (`field1`) VALUES (IF($myVariable == 0, NULL, $myVariable))";

Answer 3

try:
$myVariable="NULL";
.
if your code is:
$val=NULL;
mysql_query("SET #v=$val");
the MySQL got the string:
SET #v=0;
it's like using :
echo "SET #v=$val";

Answer 4

you can filter field's data in MySQL itself, using a TRIGGER:
CREATE TRIGGER table1_update BEFORE UPDATE ON table1 FOR EACH ROW SET
NEW.num=IF(NEW.num=0 ,NULL,NEW.num),
NEW.txt=IF(NEW.txt="",NULL,NEW.txt);
CREATE TRIGGER table1_create BEFORE INSERT ON table1 FOR EACH ROW SET
NEW.num=IF(NEW.num=0 ,NULL,NEW.num),
NEW.txt=IF(NEW.txt="",NULL,NEW.txt);