在PHP中为变量分配NULL：这是做什么的？

Question

In maintaining code, I'm encountering loops, where at the end of the loop several variables are set to NULL like so: $var = NULL; . From what I understand in the manual, NULL is meant mostly as something to compare against in PHP code. Since NULL has no type and is not a string or number, outputting it makes no sense.

I unfortunately cannot provide an example, but I think the NULL values are being written to a file in our code. My question is: does $var have a value after the assignment, and will echoing/writing it produce output?

EDIT: I have read the PHP manual entry on NULL . There is no need to post this: http://php.net/manual/en/language.types.null.php in a comment or answer, or top downvote me for not having RTM. Thank you!

Answer 1

[ghoti@pc ~]$ php -r '$i="foo"; print "ONE\n"; var_dump($i); unset($i); print "TWO\n"; var_dump($i); $i=NULL; print "THREE\n"; var_dump($i); print "\n"; if (isset($i)) print "Set.\n"; if (is_null($i)) print "is_null\n";'
ONE
string(3) "foo"
TWO
NULL
THREE
NULL
is_null
[ghoti@pc ~]$ 

The result of isset() will be boolean false, but the variable is still defined. The isset() function would be better named isnotnull() . :-P

Note that is_null() will also return true for a value that has never been set.

Yay PHP.

Answer 2

null is pretty much just like any other value in PHP (actually, it's also a different data type than string, int, etc.).

However, there is one important difference: isset($var) checks for the var to exist and have a non-null value.

If you plan to read the variable ever again before assigning a new value, unset() is the wrong way to do but assigning null is perfectly fine:

php > $a = null;
php > if($a) echo 'x';
php > unset($a);
php > if($a) echo 'x';
Notice: Undefined variable: a in php shell code on line 1
php >

As you can see, unset() actually deletes the variable, just like it never existed, while assigning null sets it to a specific value (and creates the variable if necessary).

A useful use-case of null is in default arguments when you want to know if it was provided or not and empty strings, zero, etc. are valid, too:

function foo($bar = null) {
    if($bar === null) { ... }
}

Answer 3

A variable could be set to NULL to indicate that it does not contain a value. It makes sense if at some later point in the code the variable is checked for being NULL .

A variable might be explicitly set to NULL to release memory used by it. This makes sense if the variable consumes lots of memory (see this question ).

Dry run the code and you might be able to figure out the exact reason.

Answer 4

Null in PHP means a variable were no value was assigned.

http://php.net/manual/en/language.types.null.php

Answer 5

It appears that the purpose of the null implementation based off of the information provide is to clear the variable.

You can unset a variable in PHP by setting it to NULL or using the function unset() .

unset() destroys the specified variables.

The behavior of unset() inside of a function can vary depending on what type of variable you are attempting to destroy.

If a globalized variable is unset() inside of a function, only the local variable is destroyed. The variable in the calling environment will retain the same value as before unset() was called.

Answer 6

Null is a special data type which can have only one value, which is itself. Which is to say null is not only a data type but also a keyword literal a variable of data type null is a variable that has no value assigned to it when a variable is created without a value it is automatically assigned a value of null this is so that whatever garbage was in that memory location before is cleared out otherwise the program may try to process it