php PHP中的静态变量

Static variables in PHP

提问by mike

I have found different information regarding static variables in PHP but nothing that actually explains what it is and how it really works.

我在 PHP 中发现了有关静态变量的不同信息，但没有任何内容能够真正解释它是什么以及它是如何工作的。

I have read that when used within a class that a static property cannot be used by any object instantiated by that class and that a static method can be used by an object instantiated by the class?

我已经读过，当在类中使用时，静态属性不能被该类实例化的任何对象使用，并且静态方法可以被类实例化的对象使用？

However, I have been trying to research what a static variable does within a function that is not in a class. Also, does a static variable within a function work somewhat like closure in javascript or am I totally off in this assumption?

但是，我一直在尝试研究静态变量在不在类中的函数中的作用。另外，函数中的静态变量是否有点像 javascript 中的闭包，还是我完全不同意这个假设？

回答by NullUserException

I have read that when used within a class that a static property cannot be used by any object instantiated by that class

我读过当在类中使用时，静态属性不能被该类实例化的任何对象使用

It depends on what you mean by that. eg:

这取决于你的意思。例如：

class Foo {
    static $my_var = 'Foo';
}

$x = new Foo();

echo $x::$my_var;  // works fine
echo $x->my_var;   // doesn't work - Notice: Undefined property: Foo::$my_var

and that a static method can be used by an object instantiated by the class???

并且静态方法可以被类实例化的对象使用？？？

Yes, an instantiated object belonging to the class can access a static method.

是的，属于该类的实例化对象可以访问静态方法。

The keyword staticin the context of classes behave somewhat like static class variables in other languages. A member (method or variable) declared staticis associated with the class and rather than an instance of that class. Thus, you can access it without an instance of the class (eg: in the example above, I could use Foo::$my_var)

static类上下文中的关键字的行为有点像其他语言中的静态类变量。声明的成员（方法或变量）static与类相关联，而不是与该类的实例相关联。因此，您可以在没有类实例的情况下访问它（例如：在上面的示例中，我可以使用Foo::$my_var）

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However, I have been trying to research what a static variable does within a function that is not in a class.

Also, does a static variable within a function work somewhat like closure in javascript or am I totally off in this assumption.

但是，我一直在尝试研究静态变量在不在类中的函数中的作用。

此外，函数中的静态变量是否有点像 javascript 中的闭包，或者我完全不同意这个假设。

Outside of classes (ie: in functions), a staticvariable is a variable that doesn't lose its value when the function exits. So in sense, yes, they work like closures in JavaScript.

在类之外（即：在函数中），static变量是在函数退出时不会丢失其值的变量。所以从某种意义上说，是的，它们就像 JavaScript 中的闭包一样工作。

But unlike JS closures, there's only one value for the variable that's maintained across different invocations of the same function. From the PHP manual's example:

但与 JS 闭包不同的是，在同一函数的不同调用中，变量只有一个值。来自 PHP 手册的示例：

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

test();  // prints 0
test();  // prints 1
test();  // prints 2

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Reference: statickeyword (in classes), (in functions)

参考：static关键字（在类中），（在函数中）

回答by connec

statichas two uses in PHP:

static在 PHP 中有两个用途：

First, and most commonly, it can be used to define 'class' variables/functions (as opposed to instance variables/functions), that can be accessed without instantiating a class:

首先，也是最常见的，它可用于定义“类”变量/函数（与实例变量/函数相反），无需实例化即可访问这些变量/函数：

class A {
  public static $var = 'val'; // $var is static (in class context)
  public $other_var = 'other_val'; // non-static
}

echo A::$var; // val
echo A::$other_var // doesn't work (fatal error, undefined static variable)

$a = new A;
echo $a->var // won't work (strict standards)
echo $a->other_var // other_val

Secondly, it can be used to maintain state between function calls:

其次，它可以用来维护函数调用之间的状态：

function a() {
  static $i = 0;
  $j = 0;
  return array($i++, $j++);
}
print_r(a()); // array(0, 0)
print_r(a()); // array(1, 0)
print_r(a()); // array(2, 0)
//...

Note that declaring a variable static within a function works the same regardless of whether or not the function is defined in a class, all that matters is where the variableis declared (class member or in a function).

请注意，无论函数是否在类中定义，在函数中声明变量 static 的作用都是一样的，重要的是变量的声明位置（类成员或函数）。

回答by young

A static variable in a function is initialized only in the first call of that function in its running script.

函数中的静态变量仅在其运行脚本中第一次调用该函数时初始化。

回答by Gnanasekar S

At first i will explain what will happen if static variable is not used

首先我将解释如果不使用静态变量会发生什么

<?php
function somename() {
    $var = 1;
    echo $var . "<br />";
    $var++;
}
somename();
somename();
somename();
?>

If you run the above code the output you gets will be 1 1 1 . Since everytime you called that function variable assigns to 1 and then prints it.

如果你运行上面的代码，你得到的输出将是 1 1 1 。因为每次调用该函数时，变量都会赋值为 1，然后将其打印出来。

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Now lets see what if static variableis used

现在让我们看看如果使用静态变量会怎样

<?php
function somename() {
    static $var = 1;
    echo $var . "<br />";
    $var++;
}
somename();
somename();
somename();
?>

Now if you run this code snippet the output will be 1 2 3.

现在，如果您运行此代码片段，输出将是 1 2 3。

Note:Statickeeps its value and stick around everytime the function is called. It will not lose its value when the function is called.

注意：每次调用函数时，静态都会保持其值并保持不变。当函数被调用时它不会失去它的价值。

回答by li bing zhao

class Student {
static $total_student = 0;
static  function add_student(){
    return Student::$total_student++;
}
}

First: for the add_student function, the best practice is to use static not public. Second: in the add_student function, we are using Student::$total_student,not use $this->total_student. This is big different from normal variable. Third:static variable are shared throughout the inheritance tree.

第一：对于 add_student 函数，最佳实践是使用 static 而不是 public。第二：在add_student函数中，我们使用的是Student::$total_student，而不是$this->total_student。这与正常变量有很大不同。第三：静态变量在整个继承树中共享。

take below code to see what is the result:

用下面的代码看看结果是什么：

class One {
static $foo ;
}
class Two extends One{}
class Three extends One{}
One::$foo = 1;
Two::$foo = 2;
Three::$foo = 3;
echo One::$foo;
echo Two::$foo;
echo Three::$foo;`