In PHP, what is the differences between NULL and setting a string to equal 2 single quotes

Question 1

I used to set things like this when I wanted blank values.

$blankVar = '';

Then after some months, I decided this looked better and had a clearer intent.

$blankVar = null;

This worked without hiccup for a while, but recently with a PDO prepared statements I ran into a problem. Binding a value to null made the query fail, whilst binding it to '' did not. I needed to bind it to null, so that if a condition was met, it would insert blank data.

What are the differences between the 2? I still think equaling null (or at least a constant) looks better, so should I do this?

define('EMPTY', '');

Question 2

Null is just another datatype in PHP, which has only one value (null). Since PHP is a loosly typed language, it can be confusing how it handles different values.

"", 0, "0", False, array(), Null are all considered False in PHP.

Null, however, is a different kind of animal. The main incompatibility with using Null is that you cannot tell if it isset().

$x = false;
isset($x)  ->  true
echo $x    ->  ""

$y = null;
isset($y)  ->  false
echo $y    ->  ""

//$z is not set
isset($z)  ->  false
echo $z    ->  E_NOTICE

So null is odd in the sense that it doesn't follow normal variable rules in PHP (at least some). In most cases, it is fine.

When it comes to database columns, PHP's NULL has no place there. You see, SQL is a string based language. SQL's NULL must be represented by NULL with no quotes.

So if you want an EMPTY field, set it to ""

INSERT INTO foo SET bar = ""

But if you want a NULL field, set it to NULL

INSERT INTO foo SET bar = NULL

BIG DIFFERENCE.

But if you try to insert the PHP NULL directly, it will add zero characters to the query, (which leaves you with a blank or syntax error, depending on if you quoted it).

Question 3

is_null($a) is same as $a === null.

($a === null is bit faster than is_null($a) for saving one function call, but it doesn't matter, just choose the style you like.)

For the difference of === and ==, read PHP type comparison tables

$a === null be true only if $a is null.

But for ==, the below also returns true.

null == false
null == 0
null == array()
null == ""

Question 4

Consider this script:

$arr = array();
for ($i = 0; $i < 100000; $i++) $arr[] = null;
echo memory_get_usage() . "n";

which on my machine outputs: 21687696, that is 21 MB of used memory. On the other hand using this:

$master_null = null;
$arr = array();
for ($i = 0; $i < 100000; $i++) $arr[] = $master_null;
echo memory_get_usage() . "n";

outputs: 13686832, which is 13 MB. Based on this information, you can assume that at far as memory usage is your concern, it is actually better to indeed use the "master null" variable. However you still need to have all the items in the array, and every entry in a HashTable (internal representation of arrays) takes also some memory.

If you want to dig deeper in the zvals and references, I suggest using the function debug_zval_dump. Using it, you can see, which variables share the same zval:

$a = $b = $c = $d = "abc";
debug_zval_dump($a);
$x = $y = $z = $w = null; 
debug_zval_dump($x);
$q = null;
debug_zval_dump($q);

which outputs:

string(3) "abc" refcount(5)
NULL refcount(5)
NULL refcount(2)

And this implies that although variables $x and $q are both NULL, they are not the same zval. But $x and $y share the same zval, because they are assigned to each other. I believe you know of the function debug_zval_dump, but if not, make sure you carefully read the refcount explanation at http://php.net/manual/en/function.debug-zval-dump.php.

Also at the end of my post, I want to say that this information might be useful for a better knowledge of PHP internals, I think it is quite useless to do any optimizations. Mostly because there are much better places to start optimizing scripts than such micro-optimizations. Also while this is not part of the specification, PHP authors might change this behaviour in the future (e.g. all NULL variables could share the same zval in some future version).

Question 5

In JavaScript, undefined means a variable has been declared but has not yet been assigned a value, such as:

var testVar;
alert(testVar); //shows undefined
alert(typeof testVar); //shows undefined

null is an assignment value. It can be assigned to a variable as a representation of no value:

var testVar = null;
alert(testVar); //shows null
alert(typeof testVar); //shows object

From the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.

null === undefined // false
null == undefined // true
null === null // true

and

null = 'value' // ReferenceError
undefined = 'value' // 'value'

Question 6

Well, null is not an instance of any type. Rather, it is an invalid reference.

However, System.DbNull.Value, is a valid reference to an instance of System.DbNull (System.DbNull is a singleton and System.DbNull.Value gives you a reference to the single instance of that class) that represents nonexistent^* values in the database.

^{*We would normally say null, but I don't want to confound the issue.}

So, there's a big conceptual difference between the two. The keyword null represents an invalid reference. The class System.DbNull represents a nonexistent value in a database field. In general, we should try avoid using the same thing (in this case null) to represent two very different concepts (in this case an invalid reference versus a nonexistent value in a database field).

Keep in mind, this is why a lot of people advocate using the null object pattern in general, which is exactly what System.DbNull is an example of.

In PHP, what is the differences between NULL and setting a string to equal 2 single quotes

Answers

php,null