What is the use of use in php anonymous function

Anonymous functionThe function of use is to inherit variables from the parent scope.

The following example is the most common usage. If use is not used, the variable $msg will not be found in the function.

<?php
$msg = [1,2,3];
$func = function()use($msg){
    print_r($msg);
};  

$func();
?>

Run output

Array
(
    [0] => 1
    [1] => 2
    [2] => 3
)

About the timing of inheriting variables

The behavior of inheriting variablesIs it generated when the function is defined or Generated when a function is called? Let’s adjust the order of the code in the above example and place $msg after the function definition.

<?php
$func = function()use($msg){
    print_r($msg);
};  

$msg = [1,2,3];

$func();
?>

Run output

PHP Notice:  Undefined variable: msg in /search/ballqiu/c.php on line 4

It can be seen that the behavior of inheriting variables is generated when the function is defined. In the above example,

msg is defined, so $msg is the un

defined variable when the function is run.

---

About using reference in usePassing valueWe know that if you use reference to pass value in use of an anonymous function, then Changes to parameter values ​​in anonymous functions will also affect the corresponding external variables. For example, the following example:

<?php
$msg = [1,2,3];
$func = function()use(&$msg){
    $msg[0]++;
    print_r($msg);
};


$func();

print_r($msg);
?>

Run output

Array
(
    [0] => 2
    [1] => 2
    [2] => 3
)
Array
(
    [0] => 2
    [1] => 2
    [2] => 3
)

So in any case, if you want to change the value of an external variable through an anonymous function, you must pass the value to use by reference? Look at the following example:

<?php
$msg = new ArrayObject([1,2,3], ArrayObject::ARRAY_AS_PROPS);
$func = function()use($msg){
    $msg[0]++;
    print_r($msg);
};

$func();
print_r($msg);
?>

Run output

ArrayObject Object
(
    [storage:ArrayObject:private] => Array
        (
            [0] => 2
            [1] => 2
            [2] => 3
        )

)
ArrayObject Object
(
    [storage:ArrayObject:private] => Array
        (
            [0] => 2
            [1] => 2
            [2] => 3
        )

)

It can be seen that if you pass a variable of type

object class

, even if you do not display the use of reference passing, the variable value in the anonymous function Changes also affect external related variables.
But, the problem came again. When passing an object variable to use, is there any difference between using a reference and not using a reference? Let’s look at the example

<?php
$func = function()use($msg){
    echo $msg[0],"\n";
};

$msg = new ArrayObject([1,2,3], ArrayObject::ARRAY_AS_PROPS);
$func();
?>

We use reference passing instead

$func = function()use(&$msg){
    echo $msg[0],"\n";
};

运行输出1

It can be seen that when using reference passing, even if the variable lags behind the function definition, the corresponding external variable can still be found inside the function and will not appear The variable is undefined. There is still a difference between the two.

---

About this and use in the anonymous function in the class

<?phpclass C{
    protected $_num = 0;    public function mkFunc(){
        $func = function(){
            echo $this->_num++, "\n";
        };        return $func;
    }    public function get(){
        echo $this->_num,"\n";
    }
}$obj = new C();$func = $obj->mkFunc();$func();$obj->get();?>
运行结果01

It can be seen that this in the anonymous function refers to the current

object

, no You can find it directly if you need to use use. Still the above example, what will be the effect if you must use use?

Change mkFunc to

public function mkFunc(){
    //唯一改动是此处加了use
    $func = function()use($this){
        echo $this->_num++, "\n";
    };    return $func;
}

运行输出
PHP Fatal error:  Cannot use $this as lexical variable

Modify it to

public function mkFunc(){
    $self = $this;    $func = function()use($self){
        echo $this->_num++, "\n";
    };    return $func;
}

运行结果01

It can be seen that the effect is the same whether use is used.

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