Note: empty array is converted to null by non-strict equal '==' comparison. Use is_null() or '===' if there is possible of getting empty array.
$a = array();
$a == null <== return true
$a === null < == return false
is_null($a) <== return false
特別な null
値は、ある変数が値を持たないことを表します。null
は、null 型の唯一の値です。
変数は、以下の場合に null
とみなされます。
定数 null
が代入されている場合。
まだ値が何も代入されていない場合。
unset() されている場合。
null
へのキャスト
この機能は PHP 7.2.0 で 非推奨になります。この機能に頼らないことを強く推奨します。
(unset) $var
を使って変数を null にキャストすると、
その変数を削除したり値の設定を解除したりはしません。
単に null
値を返すだけです。
Note: empty array is converted to null by non-strict equal '==' comparison. Use is_null() or '===' if there is possible of getting empty array.
$a = array();
$a == null <== return true
$a === null < == return false
is_null($a) <== return false
I would like to add for clarification that:
$x=NULL;
--$x;
// $x is still NULL.
// Decrementing NULL, using Decrement Operator, gives NULL.
$x-=1;
// $x is now int(-1).
// This actually decrements value by 1.
On the other hand, Incrementation works simply as expected.
Hope this helps :)
NULL is supposed to indicate the absence of a value, rather than being thought of as a value itself. It's the empty slot, it's the missing information, it's the unanswered question. It's not a jumped-up zero or empty set.
This is why a variable containing a NULL is considered to be unset: it doesn't have a value. Setting a variable to NULL is telling it to forget its value without providing a replacement value to remember instead. The variable remains so that you can give it a proper value to remember later; this is especially important when the variable is an array element or object property.
It's a bit of semantic awkwardness to speak of a "null value", but if a variable can exist without having a value, the language and implementation have to have something to represent that situation. Because someone will ask. If only to see if the slot has been filled.