PHP中未初始化的变量

It is not necessary to initialize variables in PHP however it is a
very good practice. Uninitialized variables have a default value of
their type depending on the context in which they are used – booleans
default to FALSE,integers and floats default to zero,strings (e.g.
used in echo) are set as an empty string and arrays become to an empty
array.

我正在玩未经初始化的高尔夫变量,但该计划没有达到我的预期.经过检查,我注意到这种奇怪的行为(所有使用的变量都是未初始化的)：

php > $a = $a + 1;
PHP Notice:  Undefined variable: a in php shell code on line 1
php > $b = $b - 1;
PHP Notice:  Undefined variable: b in php shell code on line 1
php > $c++;
PHP Notice:  Undefined variable: c in php shell code on line 1
php > $d--;
PHP Notice:  Undefined variable: d in php shell code on line 1
php > var_dump($a);
int(1)
php > var_dump($b);
int(-1)
php > var_dump($c);
int(1)
php > var_dump($d);
NULL

??1,– 1,按照手册中的说明进行操作.但是,– 没有.

之后,$a,$b和$c可用于计数.但是$d–;,不会改变$d的值,因为$d是NULL.

为什么$d设置为NULL,而不是-1？

使用前缀运算符产生相同的结果,顺便说一下：对于$v,变量设置为1;但对于 – $v;为NULL