What is the use of php callback function?

It is easier to understand it as a variable, such as $a=function(){echo "aaa";};$aa(); or function a(){}; assign the function name as a variable; $b ="a"; $b(); One is anonymous function to be called as a variable, and the other is the function name to be called as a variable. This is the common way of callback function, simple For example, this way of calling a function is called a callback function

Callback function callback

We all know that when asynchronous requests are often processed in js, if an ordinary function is used , may not be guaranteed to be called after the asynchronous request is completed. So there is a callback function, especially during file processing and ajax processing, the callback function is very useful.
Function call_user_func()
mixed call_user_func (callable $callback [, mixed $parameter [, mixed $... ]] )

return returns a composite type value, which is actually returned by the callback function Value

callable $callback The callback function can be a function name, an array, or a string

mixed $parameter The parameters of the function are displayed in sequence

Let’s introduce the use of demo
demo1: Callback php function string format

<?php
$data = array("name"=>"callback" , "value"=>"test");
$rs1 = http_build_query($data);      //直接调用php函数
$rs2  = call_user_func("http_build_query",$data); //使用回调函数
echo $rs1;  //name=callback&value=test
echo "<br />";
echo $rs2;  //name=callback&value=test
// 这里需要注意的是,参数1必须是可使用的函数可以通过function_exists()返回true的函数，这里提醒isset,empty,is_null 等认为的常用函数实际上是一个操作符.并不能算函数。

demo2 Callback php function function name variable format

<?php
//自定义函数
function myUrldecode($str){
    return urldecode($str);
}
$data = array("name"=>"callback" , "value"=>"天才");
$str  = http_build_query($data); 
$rs1  = urldecode($str);      //urlencode编码
$rs2  = call_user_func(myUrldecode,$str);
echo $rs1;  //name=callback&value=天才
echo "<br />";
echo $rs2;  //name=callback&value=天才
//这里我们可以看到,我们直接使用函数的名称也是可以的,不需要带引号字符串。
?>

demo3 Callback class method array format

<?php 
class MyClass{
    private $demo = "test";

    function myUrldecode($str){
        return urldecode($str);
    }
    static  function myUrlencode($str){
        return urlencode($str) ;
    }
}
$str = "?query=/test/demo1";

$encode  = call_user_func(array(MyClass,"myUrlencode"),$str);
//直接使用类的静态方法 将字符串进行url编码 不再是字符串或者函数名,而是一个数组格式,第一个项表示类名,第二个项则表示方法名。 第一项可以为类的引用地址,第二项为静态方法名称
$decode  = call_user_func(array("MyClass","myUrlencode"),$encode);
//同样是使用类的方法,不过调用的是普通方法名称。
echo $encode;  //%3Fquery%3D%2Ftest%2Fdemo1
echo "<br />"; //?query=/test/demo1
echo $decode;
//注意 使用方法名也具有作用域的概念,即private protected 和 public,通常回调类方法都只能调用publi 和默认作用域的 方法。
//同时如果是普通方法,并且内部使用了$this变量,那么进行调用是无法成功的.

?>

demo4 Callback class method string format

<?php
class MyClass{
    private $demo = "test";

    function myUrldecode($str){
        return urldecode($str);
    }

    private function myUrldecode2($str){
    return urldecode($str);
    }
    static  function myUrlencode($str){
        return urlencode($str) ;
    }
}

$str = "?query=/test/demo1";
  $encode  = call_user_func("MyClass::myUrlencode",$str);
  $decode  = call_user_func("MyClass::myUrldecode",$encode);
  echo $encode; //  %3Fquery%3D%2Ftest%2Fdemo1
  echo "<br />";
  echo $decode; //  ?query=/test/demo1
  $encode2  = call_user_func("MyClass::myUrlencode2",$str);
  var_dump($encode2);  // null
//如果直接使用字符串的方法的话,那么必须在类和方法名中添加::作为分割。
//这里我们发现不是静态方法也可以用::进行调用
//这里进行了一个测试,发现调用private 作用域的方法返回的是一个null值,说明确实存在作用域的关系
?>

demo5 Callback object method array format

<?php
class MyClass{
    private $demo = "test";


    function myUrldecode($str){
        return urldecode($str)  ."-" .$this->demo ; //调用内部的this作用域
    }
    static  function myUrlencode($str){
        return urlencode($str);
    }
}

 $str = "?query=/test/demo1";
 $class =  new MyClass();
 $encode  = call_user_func(array($class,"myUrlencode"),$str);
 $decode  = call_user_func(array($class,"myUrldecode"),$str);
 echo $encode; //%3Fquery%3D%2Ftest%2Fdemo1
 echo "<br />";
 echo $decode; //?query=/test/demo1-test
 //很明显,如果使用对象做为回调函数,内部的private 属性和方法也可以使用,但是对外的方法必须为默认或者 public类型
 //对象数组方式第一个选项必须为一个对象
?>

Sometimes we find that the parameters passed may not be fixed, then use call_user_func () will cause some trouble. At this time, you can use the call_user_func_array() method. Most of the usage methods are the same, but the use of parameters will be slightly different.
Function call_user_func_array()

demo1 Callback php function with variable parameters

<?php 
   $record1 = array(
            &#39;id&#39; => 2135,
    );
    $record2 = array(
        &#39;first_name&#39; => &#39;Sally&#39;,
    );
    $record3 = array(
        &#39;last_name&#39; => &#39;Jones&#39;,
    );
$return  = array_merge($record1,$record2,$record3);
//如果使用回调函数,则如下
$return  = call_user_func_array("array_merge", array($record1,$record2,$record3));
print_r($return); //Array ( [id] => 2135 [first_name] => Sally [last_name] => Jones )
//一些特别的函数允许你有不固定的参数,如果你还将它作为回调函数,则可以使用这个方法进行调用.
?>

tips1 How to determine whether it is a callback function
bool is_callable (callable $name [, bool $syntax_only = false [, string &$callable_name ]] )

name
The callback function to be checked.
syntax_only
If set to TRUE, this function only verifies that name may be a function or method.
callable_name
Accepts "callable name".
return
If name is callable, return TRUE, otherwise return FALSE

<?php
// is_callable(); 
$bool = is_callable(urlencode);  // true
$bool = is_callable(urlencode,false);  // true
$bool = is_callable(urlencode,true);  // true
$bool = is_callable("urlencode",false,$callable_name1);
// string urlencode
$bool = is_callable("urldecode",true,$callable_name2); 
// string urldecode 即使不存在回调函数也会返回true
?>

tips2 Get the parameters of the function, similar to arguments

<?php
//这里指的是非客户端php脚本
function sum($first,$second){
$args = func_get_args(); //获取列表  
//func_get_arg($index) 则获取指定的位置的参数
$len  = func_num_args(); //获取长度
$sum = 0 ; 
foreach($args as $key=>$val){
 $sum += (int) $val; 
}
return $sum;
}
$rs = sum(1,2,3,4,5);
var_dump($rs); //  int 15
//注意: 在sum函数中修改了$first,$second的变量,同时又不是引用对象 变量的话,则不会修改
?>

in js

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