值不丢失，虽然仅在局部函数中存在

w

http://php.net/manual/en/language.variables.scope.php

http://php.net/manual/zh/language.variables.scope.php 

<?php
function test()
{
    $a = 0;
    echo $a;
    $a++;
}

test();
test();
test();
test();

<?php
function test()
{
    static $a = 0;
    echo $a;
    $a++;
}

test();
test();
test();
test(); 

//0000

//0123

<?php
function test()
{
    static $a = 0;
    echo $a;
    $a++;
}

test();
test();
test();
test();
echo $a;

//0123 Notice: Undefined variable: a

变量范围的另一个重要特性是静态变量（static variable）。静态变量仅在局部函数域中存在，但当程序执行离开此作用域时，其值并不丢失。
本函数没什么用处，因为每次调用时都会将 ￥a 的值设为 0 并输出 0。将变量加一的 ￥a++ 没有作用，因为一旦退出本函数则变量 ￥a 就不存在了。要写一个不会丢失本次计数值的计数函数，要将变量 ￥a 定义为静态的：
现在，变量 ￥a 仅在第一次调用 test() 函数时被初始化，之后每次调用 test() 函数都会输出 ￥a 的值并加一。

Another important feature of variable scoping is the static variable. A static variable exists only in a local function scope, but it does not lose its value when program execution leaves this scope. Consider the following example:

This function is quite useless since every time it is called it sets ￥a to 0 and prints 0. The ￥a++ which increments the variable serves no purpose since as soon as the function exits the ￥a variable disappears. To make a useful counting function which will not lose track of the current count, the ￥a variable is declared static:

Now, ￥a is initialized only in first call of function and every time the test() function is called it will print the value of ￥a and increment it.

<?php
function test()
{
    static $count = 0;
   // echo $count;
    $count++;
    echo $count;
    if ($count < 10) {
        test();
    }
    echo 'w', $count;
    $count--;
}

test();
die();
test();
test();
test();