What is a callback function? How is it defined? what is the function? (detailed explanation and examples)

The previous article introduced you to "What is a variable function in PHP? How to use variable functions? (Share) 》, this article continues to introduce to you what is a callback function? How is it defined? what is the function? (Detailed explanation and examples) Friends in need can refer to it. I hope it will be helpful to everyone.

What is a callback function:

A callback function, or callback for short, refers to a reference to a certain block of executable code that is passed to other code through function parameters. The callback function is not called directly by the implementer of the function, but is called by another party when a specific event or condition occurs to respond to the event or condition. This design allows low-level code to call subroutines defined at a higher level. The form of callbacks varies from programming language to programming language.

To put it simply, the user is the implementer, and the implementer needs to call the A() function. However, in order for the A() function to be universal, it needs to call the function B() provided by the implementer according to the wishes of the implementer. Here B() is the callback function. Callback functions are widely used in Windows programming.

Let’s take the specific code as an example:

<?php
/**** * * *回调函数************/
//定义函数
    function write($func){ 
//变量函数的出现
             $func();
}
//定义一堆函数
    function writeA( ){
         echo&#39; 风轻云淡,岁月安好 <br>&#39; ;
}
    function writeB( ){
         echo &#39;孤独从一开始注定要用一生来承担 <br>&#39; ;
}
    function writeC(){
         echo &#39; 任何一件事情,只要心甘情愿,总是能够变得简单 <br>&#39; ;
}
//回调函数的出现
     write(&#39;writeA&#39;);
     write(&#39;writeB&#39;);
     write(&#39;writeC&#39;);
     ?>

Code analysis:

First we define a function function write(); at this time the variable function appears, for the callback function, We also define a push function first. As shown in the above code, I call it through the write function and pass out a string. Then this string will be assigned to the variable ($func). When we run the result, the result will appear. It is 'The wind is gentle and the clouds are calm, and the years are peaceful', (the code result is as follows), similarly, we follow this analogy and output writeB, writeC, in sequence (the code result is as follows), so we can know that through the variable function , call other functions later, this is the callback function.

Example 2:

<?php
/**** * * *回调函数************/
//定义函数
    function write($func){ 
//变量函数的出现
             $func();
}
//定义一堆函数
    function writeA( ){
         echo&#39; 风轻云淡,岁月安好 <br>&#39; ;
}
    function writeB( ){
         echo &#39;孤独从一开始注定要用一生来承担 <br>&#39; ;
}
    function writeC(){
         echo &#39; 任何一件事情,只要心甘情愿,总是能够变得简单 <br>&#39; ;
}
//回调函数的出现
     write(&#39;writeA&#39;);
     write(&#39;writeB&#39;);
     write(&#39;writeC&#39;);
     echo &#39;<hr>&#39;;
     writeC();
     ?>

Of course, we do not use the callback function, directly output write(), and can also output its results; code results As shown below;

##Based on the above, we once again summarize what a callback function is:

function a($var){
$var(); //回调函数要素1,变量函数
}
a( 'b )//在a函数中调用b函数回调函数要素2. 传入函数名称字符串。

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