Summary of several PHP anonymous function usage examples

I unknowingly discovered that PHP has been released to version 5.5, and I have been using PHP5.2, which makes me look like a young man from the mountains, rustic and backward. After I got used to using closures in javascript, I suddenly became interested in PHP's closures.

So I downloaded a WAMP integrated development environment on the Internet, which is the PHP5.3 version (PHP5.3 began to introduce the closure feature). I have to say that WAMP is really convenient to install and use. . After a simple configuration, let's start.

Anonymous function
When it comes to closures, you have to think of anonymous functions, also called closure functions (closures), which look like PHP closures Implementation mainly depends on it. Declaring an anonymous function is like this:

The code is as follows:

$func = function() {
}; //带结束符

As you can see, because the anonymous function has no name, if you want to use it, you need to return it to a variable. Anonymous functions can also declare parameters like ordinary functions, and the calling method is also the same:

The code is as follows:

$func = function( $param ) 
{    
echo $param;
};
$func( 'some string' );
//输出：//some string

By the way, before PHP introduces closures, there is also one that can be created Anonymous function function: create function, but the code logic can only be written as string, which looks very obscure and difficult to maintain, so few people use it.

Implementing closure
Pass anonymous functions as parameters in ordinary functions, and they can also be returned. This implements a simple closure.

There are three examples below

The code is as follows:

//例一
//在函数里定义一个匿名函数，并且调用它
function printStr() {
    $func = function( $str ) {
        echo $str;
    };
    $func( 'some string' );
}
printStr();
 
//例二
//在函数中把匿名函数返回，并且调用它
function getPrintStrFunc() {
    $func = function( $str ) {
        echo $str;
    };
    return $func;
}
$printStrFunc = getPrintStrFunc();
$printStrFunc( 'some string' );
 

//例三
//把匿名函数当做参数传递，并且调用它
function callFunc( $func ) {
    $func( 'some string' );
}
$printStrFunc = function( $str ) {
    echo $str;
};
callFunc( $printStrFunc );
//也可以直接将匿名函数进行传递。如果你了解js，这种写法可能会很熟悉
callFunc( function( $str ) {
    echo $str;
} );

Keywords connecting closures and external variables: USE
Closures can save some variables and values ​​​​in the context of the code block. By default in PHP, anonymous functions cannot call context variables in the code block where they are located, but need to use the use keyword.

Let’s take a look at another example:

The code is as follows:

function getMoney() {
    $rmb = 1;
    $dollar = 6;
    $func = function() use ( $rmb ) {
        echo $rmb;
        echo $dollar;
    };
    $func();
}
getMoney();
//输出：
//1
//报错，找不到dorllar变量

As you can see, dollar is not declared in the use keyword, nor in this anonymous function. It cannot be obtained, so pay attention to this issue during development.

Some people may wonder whether context variables can be changed in anonymous functions, but I found that it is not possible:

The code is as follows:

function getMoney() {
    $rmb = 1;
    $func = function() use ( $rmb ) {
        echo $rmb;
        //把$rmb的值加1
        $rmb++;
    };
    $func();
    echo $rmb;
}
getMoney();
//输出：
//1
//1

Ah, it turns out that what use refers to is just a copy of the variable. But I want a full reference to the variable, not a copy.

To achieve this effect, just add an & symbol before the variable:

The code is as follows:

function getMoney() {
    $rmb = 1;
    $func = function() use ( &$rmb ) {
        echo $rmb;
        //把$rmb的值加1
        $rmb++;
    };
    $func();
    echo $rmb;
}
getMoney();
//输出：
//1
//2

Okay, so anonymous The function can then reference the context variables. If the anonymous function is returned to the outside world, the anonymous function will save the variables referenced by use, but the outside world will not be able to obtain these variables. In this way, the concept of 'closure' may be clearer.

Change the above example according to the description:

The code is as follows:

function getMoneyFunc() {
    $rmb = 1;
    $func = function() use ( &$rmb ) {
        echo $rmb;
        //把$rmb的值加1
        $rmb++;
    };
    return $func;
}
$getMoney = getMoneyFunc();
$getMoney();
$getMoney();
$getMoney();
//输出：
//1
//2
//3

Summary
The characteristics of PHP closures are not What a surprise. In fact, similar or even more powerful functions can be achieved by using CLASS. It cannot be compared with the closure of js. We can only look forward to the improvement of PHP's closure support in the future. However, anonymous functions are still quite useful. For example, when using functions such as preg_replace_callback, you don't need to declare the callback function externally.

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