Default parameter function of php function

Introduction to php default parameter function

In PHP, the default method of calling a function is supported, which is to specify a default value for the parameter. The previous section introduced the reference parameter function of php function. If the value of the parameter is not specified when calling the function, the default value of the parameter will be used in the function. The default value must be a constant expression, not a variable, class member, or function call. PHP also allows the use of arrays and the special type NULL as default parameters.

The function format of php default parameters is described as follows:

mixed funName(string name[, string value [, int expire]])    //参数列表中出现使用 [ ] 描述参数

The parameters are declared when defining the function, but no parameters or fewer parameters are specified when calling the function , a warning about missing parameters will appear.

The following is an example to illustrate the default parameters:

<?php
 //自定义一个函数person，用于打印一个人的属性
 function person($name="张三", $age=20, $sex="男"){
   echo "姓名：{$name}, 年龄：{$age}, 性别：{$sex} <br>";
 }
 person();               //全部使用默认参数
 person("李四");            //第一个默认参数被覆盖，后面两个参数为默认
 person("李四",22);          //前两个默认参数被覆盖，后面一个参数为默认
 person("李四",22,"女");        //三个默认参数都被覆盖
?>

The output result after execution of the program is as follows:

Name: Zhang San, Age: 20, Gender: Male

Name: Li Si, Age: 20, Gender: Male

Name: Li Si, Age: 22, Gender: Male

Name: Li Si, Age: 22, Gender: Female

In the above example, a function named person() with three parameters is declared, and the three parameters are all assigned initial values ​​by default, that is, Default parameters. When calling this function, if few or no parameters are passed, the default values ​​will be used for the parameters. If the user passes a value when calling the function, the passed value is used.

When calling a function to pass parameters, the actual parameters and formal parameters are passed in order. If the actual parameters are less than the formal parameters, the formal parameter on the right will not be passed. When using default parameters, any default parameters must be placed to the right of any non-default parameters, otherwise the function may not work as expected.

For example, the following function declaration is an incorrect use of the function's default parameters. The last two parameters are not passed values ​​and have no default values. A warning appears when calling.

The following is an example to illustrate:

<?php
 //自定义一个函数person，用于打印一个人的属性
 function person($name="张三", $age, $sex){
   echo "姓名：{$name}, 年龄：{$age}, 性别：{$sex} <br>";
 }
 person("李四");    //第一个默认参数被覆盖，后面两个参数没有传值，会出现两条警告信息
?>

Note: You only need to list the default parameters in the parameter list of the function header. Without default values ​​following parameters, the program will execute correctly. In the above code, when the function person() is called, the first two parameters must be passed by value. If not, an error will occur; and the last parameter is an optional parameter. If no value is passed, use The default value.

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