is_null()

(PHP 4 >= 4.0.4, PHP 5, PHP 7)

检测变量是否为NULL

描述

is_null(mixed $var): bool

如果$var是null则返回TRUE，否则返回FALSE。

查看NULL类型获知变量什么时候被认为是NULL，而什么时候不是。

参见NULL、is_bool()、is_numeric()、is_float()、is_int()、is_string()、is_object()、is_array()、is_integer()和is_real()。

Micro optimization isn't worth it.
You had to do it ten million times to notice a difference, a little more than 2 seconds
$a===NULL; Took: 1.2424390316s
 is_null($a); Took: 3.70693397522s
difference = 2.46449494362
difference/10,000,000 = 0.000000246449494362
The execution time difference between ===NULL and is_null is less than 250 nanoseconds. Go optimize something that matters.

See how php parses different values. $var is the variable.
$var    =  NULL  ""  0  "0"  1
strlen($var)  =  0  0  1  1  1
is_null($var)  =  TRUE  FALSE  FALSE  FALSE  FALSE
$var == ""  =  TRUE  TRUE  TRUE  FALSE  FALSE
!$var    =  TRUE  TRUE  TRUE  TRUE  FALSE
!is_null($var)  =  FALSE  TRUE  TRUE  TRUE  TRUE
$var != ""  =  FALSE  FALSE  FALSE  TRUE  TRUE
$var    =  FALSE  FALSE  FALSE  FALSE  TRUE
Peace!

In PHP 7 (phpng), is_null is actually marginally faster than ===, although the performance difference between the two is far smaller.
PHP 5.5.9
is_null - float(2.2381200790405)
===   - float(1.0024659633636)
=== faster by ~100ns per call
PHP 7.0.0-dev (built: May 19 2015 10:16:06)
is_null - float(1.4121870994568)
===   - float(1.4577329158783)
is_null faster by ~5ns per call

For what I realized is that is_null($var) returns exactly the opposite of isset($var) , except that is_null($var) throws a notice if $var hasn't been set yet.
the following will prove that:
<?php
$quirks = array(null, true, false, 0, 1, '', "\0", "unset");
foreach($quirks as $var) {
  if ($var === "unset") unset($var);
  echo is_null($var) ? 1 : 0;
  echo isset($var) ? 1 : 0;
  echo "\n";
}
?>
this will print out something like:
10  // null
01  // true
01  // false
01  // 0
01  // 1
01  // ''
01  // "\0"
Notice: Undefined variable: var in /srv/www/htdocs/sandbox/null/nulltest.php on line 8
10  // (unset)
For the major quirky types/values is_null($var) obviously always returns the opposite of isset($var), and the notice clearly points out the faulty line with the is_null() statement. You might want to examine the return value of those functions in detail, but since both are specified to return boolean types there should be no doubt.
A second look into the PHP specs tells that is_null() checks whether a value is null or not. So, you may pass any VALUE to it, eg. the result of a function.
isset() on the other hand is supposed to check for a VARIABLE's existence, which makes it a language construct rather than a function. Its sole porpuse lies in that checking. Passing anything else will result in an error.
Knowing that, allows us to draw the following unlikely conclusion:
isset() as a language construct is way faster, more reliable and powerful than is_null() and should be prefered over is_null(), except for when you're directly passing a function's result, which is considered bad programming practice anyways.

Using === NULL instead of is_null(), is actually useful in loaded server scenarios where you have hundreds or thousands of requests per second. Saving microseconds on a lot of "simple" operations in the entire PHP execution chain usually results in being able to serve more pages per second at the same speed, or lowering your cpu usage. People usually write very bad and slow code.

$var===NULL is much faster than is_null($var) (with the same result)
I did some benchmarking with 10 million iterations:
$a=null;
 isset($a); Took: 1.71841216087s
 $a==NULL; Took: 1.27205181122s
 $a===NULL; Took: 1.2424390316s
 is_null($a); Took: 3.70693397522s
$a=5;
 isset($a); Took: 1.15165400505s
 $a==NULL; Took: 1.41901302338s
 $a===NULL; Took: 1.21655392647s
 is_null($a); Took: 3.78501200676s
error_reporting(E_ALL&~E_NOTICE);
unset($a);
 isset($a); Took: 1.51441502571s
 $a==NULL; Took: 16.5414860249s
 $a===NULL; Took: 16.1273870468s
 is_null($a); Took: 23.1918480396s
Please note, that isset is only included because it gives a good performance in any case; HOWEVER isset is NOT the same, or the opposite.
But you might be able to use isset() instead of null-checking.
You should not use is_null, except when you need a callback-function, or for conformity with is_int, is_float, etc.

Regarding avoidance of NULLs in your MySQL queries, why not use IS NULL and IS NOT NULL in your WHERE clauses.
SELECT * 
FROM someDatabase 
WHERE someAttribute IS NOT NULL
Cheers,
Michael

Don't try to test
if ($intSomething==NULL) {
 ...
}
use is_null() instead.
The first statement misses 0 values.
Regards,
Calin
[Ed. note: this is because == tests for equivalence of value, but not type. NULL evaluates to 
false, as does 0, so NULL == 0 is true--even though 0 is type int and NULL is type null. 
You should use either is_null() as noted or ===, which returns true only if its operands are 
equal and of the same type.]