How to write php callback function

The definition and calling of ordinary functions are similar to js. This definition method does not require a return value, even if there is a return value, there is no need to add it when declaring.

In later PHP versions, a very useful feature was added, which is that it can be called before the function is defined.

echo add(1,2);
echo "
";
function add($a,$b){
    return $a+$b;
}
function sub($a,$b){
    return $a-$b;
}
echo add(23,12);
echo "
";
echo sub(23,22);
echo "
";

The following is a very useful function, which is the variable function. As the name suggests, the function is used as a variable.

The advantage is that different functions can be called with the same variable, which is very similar to polymorphic calling of functions.

$var="add";
echo $var(4,2);
echo "
";
$var="sub";
echo $var(4,2);
echo "
";

The callback function is to pass a process to a function when passing a simple parameter cannot solve the problem, so as to achieve the purpose.

Passing a function as a parameter when calling a function is a function callback.

$arr=array(2,3,5,4,1,6,7,9,8);
var_dump($arr);
echo "
";

//这里是自定义回调函数,返回-1是指将两个元素交换,0和1是不发生改变。
function myrule($a,$b){
    if ($a>$b){
        return 1;
    }
    elseif ($a<$b){
        return -1;
    }
    else{
        return 0;
    }
}
//usort就是系统函数,但是他的第二个参数是回调函数,这个函数参数就是排序规则
usort($arr,"myrule");
var_dump($arr);
echo "
";

Write your own callback function and use variable functions and callbacks to complete a simple filtering condition. If multiple conditions need to be satisfied at the same time, just give it an && relationship.

The variable function used can be called using a function called call_user_func_array() in the system. It has two parameters: the name of the callback function and the parameter array

call_user_func_array("demo ", array(1,3)); Its advantage is that the number of parameters in array can be less than that of the original function, even when there are default parameters.

//rule1除去数组中是三的倍数的数
function rule1($a){
    if ($a%3==0){
        return true;
    }else{
        return false;
    }
}
//rule2是除去数组中的回文数（从左边读与从右边读是一样的）
function rule2($a){
    if ($a==strrev($a)){
        return true;
    }else{
        return false;
    }
}
function demo($n,$var){
    for ($i=0;$i<$n;$i++){
        if (call_user_func_array($var,array(23)))
        //if ($var($i))
        {
            continue;
        }else{
            echo $i."
";
        }
    }
}
$var="rule1";
demo(100,$var);
echo "
";
echo "
";
$var="rule2";
demo(200,$var);
echo "
";

Note that when calling a method in an object, we need to pass in an anonymous object and then specify the function name

. When calling a static method in a class, we only need to specify the class name.

In the above two cases, the variable function cannot be completely used. You can only use the system callback call_user_func_array(), which is just a variable function to pass parameters without calling

class A{
    function one(){
    }
    static function two(){
    }}demo(200,array(new A,"one"));demo(200,array("A","two"));

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