Q:

MySQL, how to insert null dates

I am having trouble inserting null values into date fields into a MySQL table. 
Here is the insert query:
$query = 'INSERT INTO table (column_s1, column_s2, column_d1, column_d2)
VALUES ("'.$string1.'", "'.$string2.'", '.$date1.', '.$date2.')';

Columns s1 and s2 take string values and d1 and d2 take dates.  When I run this query with only the string fields, there is no problem.
The date values can be either set or null, so I have not included the quotation marks in the query, but have instead added them to the variable earlier on.  This is the php code I am using to set the date values:
if (empty($date1)){
    $date1 = NULL;
}
else{
    $date1part = explode("/",$date1);
    $date1 = '"'.$date1part[2].'/'.$date1part[1].'/'.$date1part[0].'"';
}

When the date values are all set, the record is inserted correctly. However, when either of the dates is null, nothing is inserted.
Why can't I just insert null values into MySQL like this?

A:

Try this:
$query = "INSERT INTO table (column_s1, column_s2, column_d1, column_d2) 
          VALUES ('$string1', '$string2', " . ($date1==NULL ? "NULL" : "'$date1'") . ", " . ($date2==NULL ? "NULL" : "'$date2'") . ");";

so for example if you put this into query:
$string1 = "s1";
$string2 = "s2";
$date1 = NULL;
$date2 = NULL;

result should be:
INSERT INTO table (column_s1, column_s2, column_d1, column_d2) VALUES ('s1', 's2', NULL, NULL);

A:

You should convert the null variable into a NULL string first
Like this:
if(is_null($date1)){
    $date1 = 'NULL';
}

If you are using a MySQL date column, you must also specify that it should hold null when creating it, like this:
CREATE TABLE `table` (
id INT NOT NULL AUTO_INCREMENT,
date DATE NULL DEFAULT NULL,
PRIMARY KEY(id)
)

It is also very important that you perform the query with bound parameters, for example using pdo

http://www.php.net/manual/en/pdo.construct.php
http://php.net/manual/en/pdo.prepared-statements.php
How do I insert NULL values using PDO?

Something like this:
$query = 'INSERT INTO table (column_s1, column_s2, column_d1, column_d2)
VALUES (?, ?, ?, ?)';
$stmt = $db->prepare($query);
$stmt->execute(array($string1,$string2,$date1,$date2));

