What is the concept of callback function? , how to use custom callback function in php?

UserCustom functions are also called custom functions. They are not provided by PHP and are created by programmers. Since you create such functions yourself, you can fully control these functions. Therefore, you can Let a function run exactly the way you want.

1. Declaring a function in PHP. The method of defining a function is almost the same as in otherprogramming languages. The following is the syntax structure of PHP declaring a function :

Function function_name($argument1,$argument2,$argument3,...$argumentn){//Function code codeReturn return value;} In the above syntax structure, the meaning of the keywords is as follows.

(1)function: Keyword used to declare user-defined functions.

(2)function_name: The name of the function to be created. This name will be used when it is called later. Function name It should be unique because PHP does not support overloading. When naming functions, you need to follow the same principles as naming variables. But the function name cannot start with $, while the variable can be

. (3) argument: to be passed The value given to the function. The function can have multiple parameters, with commas between them. But the parameter items are optional, and you can not pass any parameters when calling the function.

(4)code: is in A piece of code that is executed when the function is called. If there are two or more statements, the code must be enclosed in braces "{}". However, if there is only one code, no braces are needed.

(5)Return: Return the value required by the calling code. Any type can be returned, including lists and objects. This causes the function to immediately end its execution and pass control back to the line where it was called

.2, parameterless function

Code func_1.php

<?phprequire &#39;a.php&#39;;
echo "我不会被执行!";
?>

3, parameterized function PHP supports passing parameters by value (default), passing by reference and default parameter values. Variable The length parameter list is only supported in PHP4 and subsequent versions. (1) Value passing parameters. Parameter passing by value is the default passing method of PHP. Using this method, a value (parameter) must be passed when the main program is called ).

Code sum.php

<?php
function sum ($a,$b)
{
     Echo $a+$b;
}
sum(100,20);   //开始调用该函数
?>

(2) Reference parameters. When passing by value, only a copy of the parameter is passed to the called function. But when it is called Any modifications to these values ​​within the function will not affect the original values ​​in the calling function. Passing by reference is actually address passing, passing the address of a variable as a parameter. Code

valuechange.php

<?php
$myNum=100;
function Valuechange ($number)
{
     $number=$number+1;
Echo $number . "<br>";
}
Valuechange($myNum);
Echo $myNum;
?>

(3)Default value parameter. Using the default parameter value passing method, the function must have a parameter when called. If there is no value used, the default value is passed to function parameter. The default value must be a constant expression, not a variable, class member or function call. Tip: When using default parameters, any default parameters must be placed to the right of the non-default parameters; otherwise, the function may not Works as expected.

Code func_default.php

<?php
$myNum=100;
function Valuechange ($number)
{
     $number=$number+1;
Echo $number . "<br>";
}
Valuechange($myNum);
Echo $myNum;
?>

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