value評価する変数。
例1 is_null() の例
<?php
error_reporting(E_ALL);
$foo = NULL;
var_dump(is_null($inexistent), is_null($foo));
?>Notice: Undefined variable: inexistent in ... bool(true) bool(true)
null 型Micro optimization isn't worth it.You had to do it ten million times to notice a difference, a little more than 2 seconds$a===NULL; Took: 1.2424390316s is_null($a); Took: 3.70693397522sdifference = 2.46449494362difference/10,000,000 = 0.000000246449494362The 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" 1strlen($var) = 0 0 1 1 1is_null($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$var = FALSE FALSE FALSE FALSE TRUEPeace!In PHP 7 (phpng), is_null is actually marginally faster than ===, although the performance difference between the two is far smaller.PHP 5.5.9is_null - float(2.2381200790405)=== - float(1.0024659633636)=== faster by ~100ns per callPHP 7.0.0-dev (built: May 19 2015 10:16:06)is_null - float(1.4121870994568)=== - float(1.4577329158783)is_null faster by ~5ns per callA quick test in 2022 on PHP 8.1 confirms there is still no need to micro-optimize NULL checks:<?php// Comparison Operator$before = microtime(true);$var = null;for ($i=0 ; $i<1000000000 ; $i++) { if($var === null) {}}$after = microtime(true);echo ' ===: ' . ($after - $before) . " seconds\n";// Function$before = microtime(true);$var = null;for ($i=0 ; $i<1000000000 ; $i++) { if(is_null($var)) {}}$after = microtime(true);echo 'is_null: ' . ($after - $before) . " seconds\n";// ===: 4.1487579345703 seconds// is_null: 4.1316878795624 secondsFor what I realized is that is_null($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($var) ? 1 : 0; echo "\n";}?>this will print out something like:10 // null01 // true01 // false01 // 001 // 101 // ''01 // "\0"Notice: Undefined variable: var in /srv/www/htdocs/sandbox/null/nulltest.php on line 810 // (unset)For the major quirky types/values is_null($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.