After a constant value is defined, it cannot be changed anywhere else in the script.
PHP Constants
A constant is an identifier of a simple value. This value cannot be changed in the script.
A constant consists of English letters, underscores, and numbers (it is recommended to only use letters and underscores), but numbers cannot appear as the first letter. (The $ modifier is not required on the constant name).
Constant names can be lowercase, but are usually uppercase.
Constant names may not be quoted, but are usually quoted.
When calling a constant in a string, it must be outside the quotation marks.
Constants can be used throughout the script.
Set constants using the define() function. The function syntax is as follows:
bool define ( string $name , mixed $value [, bool $case_insensitive = false ] )
This function has three parameters:
name: required parameter, constant name, that is, identifier.
value: required parameter, value of constant.
case_insensitive: Optional parameter, if set to TRUE, this constant is case insensitive. The default is case-sensitive.
can be simply understood as:
define (constant name, constant value)
In the following example, we create a size-sensitive Write a constant, the constant value is "Welcome to php.cn":
<?php
// 区分大小写的常量名
define("GREETING", "欢迎访问 php.cn");
echo GREETING; // 输出 "欢迎访问 php.cn"
echo '<br>';
echo greeting; // 输出 "greeting"
?> In the following example, we create a case-insensitive constant, the constant value is "Welcome to php.cn":
<?php
// 不区分大小写的常量名
define("GREETING", "欢迎访问 php.cn", true);
echo greeting; // 输出 "欢迎访问 php.cn"
?>Constants are global
After a constant is defined, it defaults to a global variable and can be used anywhere in the entire running script.
The following example demonstrates the use of constants within a function. Even if the constant is defined outside the function, the constant can be used normally.
<?php
define("GREETING", "欢迎访问 php.cn");
function myTest()
{
echo GREETING;
}
myTest(); // 输出 "欢迎访问 php.cn"
?>We will learn about the definition of functions later. For related information, please refer to PHP function
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