Detailed explanation of PHP constants

1. Declare and use constants

Constants can be understood as unchanging quantities. Once a constant is defined, it cannot be changed anywhere else in the script. A constant consists of English letters, underscores, and mathematics. But numbers cannot appear as initial letters. D Use the define ()

function

to define the constant in PHP. The grammar format is as follows:

 define(string constant_name,mixed value,case_sensitive=true)

Parameter description:

constant_name must choose parameters. Constant name, i.e. identifier.

Value Required parameter. The value of the constant.

case_sensitive optional parameter. Specify whether to be case sensitive. Set to true to indicate insensitivity.

There are two ways to obtain constants: one is to use constants to obtain them directly; the other is to use the constant() function. The constant() function has the same output effect as directly using the constant name. But the function can dynamically output different constants, making it more flexible and convenient to use. The function syntax format is:

   mixde constant(string const_name)

The parameter const_name is the name of the constant to be obtained, or it can be a

variable

that stores the constant name. If the acquisition is successful, the constant value is returned, otherwise an error message is displayedThe constant is not defined. To determine whether a constant has been defined, you can use the define() function. The function syntax format is:

  bool defined(string constant_name)

The parameter constant_name is the name of the constant to be obtained. It returns true if successful, otherwise it returns false.

Example:

In order to better understand how to define constants, here is an example of defining constants. The above three functions, define() function, constant() function and defined() function, are used in the example. Use the defined() function to define a constant, use the constant() function to dynamically obtain the value of the constant, and use the define() function to determine whether the constant is defined. The code is as follows:

<?php
/define("变量名",值,是否模糊大小写)
define("MESSAGE","能看到一次");
echo MESSAGE."<br >";
echo Message."<br >";    //输出Message，表示没有该常量
define("COUNT","能看到多次",true);
echo COUNT."<br >";
echo CoUnT."<br >";      //输出COUNT，设定大小写不敏感
$name = "count";
//通过constant()函数获取常量
echo constant($name)."<br >";
//是否存在常量 "MESSAGE"
echo (defined("MESSAGE"))."<br >";  //如果常量被定义，则返回true，使用echo输出显示1。

Output result:

能看到一次
Notice: Use of undefined constant Message - assumed 'Message' in 编辑器代码错误位置。
Message
能看到多次
能看到多次
能看到多次
1

2. Predefined constants You can use predefined constants in php to obtain information in php. The predefined constants are as follows:

Constant name __FILE____LINE__PHP_OSPHP_VERSIONTRUEFALSENULLE_ERRORE_WARNINGE_PARSEE_NOTICENote: The "__" in FILE and LINE are two underscores , instead of a "_".

says that
default constant is the PHP program file name
default constant, It is the number of lines of PHP program
Built-in constant refers to the name of the operating system that executes the PHP parser
Built-in constant is the version of the PHP program
This constant is a true value
This constant is a false value
A null value
error, causing the php script to terminate
warning, which will not cause php Script execution terminates
Parse errors, reported by the program parser
Non-critical errors, such as variables not initialized

Explanation: The predefined constants starting with E_ are the error debugging part of PHP.

Example:

There is no difference in usage between predefined constants and user-defined constants. The following uses predefined constants to output information in PHP.

<?php
echo "PHP程序当前文件路径名:".FILE;
echo "<br >PHP程序当前行数:".LINE;
echo "<br >当前PHP程序的版本:".PHP_VERSION;
echo "<br >当前操作系统:".PHP_OS;

Output results similar to the following:

PHP程序当前文件路径名:D:\phpStudy\www\index.php
PHP程序当前行数:3
当前PHP程序的版本:5.6.27
当前操作系统:WINNT

Note: Depending on the operating system and software version used by each user, the results may not be the same.