Backend DevelopmentPHP TutorialDetailed explanation of php custom/system/class constant code examplesDetailed explanation of php custom/system/class constant code examples
Let’s look at a piece of code first:
<span style="font-size:18px;">
<?php
// php常量:分为以下四种
//（1）、自定义常量：
// 必须用函数define()定义
// define() 函数 - 它使用三个参数：
// 首个参数定义常量的名称
// 第二个参数定义常量的值
// 可选的第三个参数规定常量名是否对大小写敏感。默认是 false，对大小写敏感。
// 定义完后其值不能再改变了
// 使用时直接用常量名，不能像变量一样在前面加$s
// 实例代码：
define("GREETING", "Welcome to W3School.com.cn!");
echo "对大小写敏感：".GREETING."<br/>";
//第三个参数为 true ，对大小写不敏感
define("GREETING", "Welcome to W3School.com.cn!", true);
echo "对大小写不敏感：".greeting."<br/>";
//（2）、系统常量：
// PHP_VERSION:当前解析器的版本号
// PHP_OS：执行当前PHP版本的操作系统名称
// 可以直接拿来使用，例如要查看执行当前PHP版本的操作系统名称，就可以写成 echo PHP_OS
// 实例代码：
echo "<br/>";
//输出当前解析器的版本号
echo "当前解析器的版本号是: ".PHP_VERSION;
echo "<br/>";
//输出执行当前PHP版本的操作系统名称
echo "当前操作系统是： ".PHP_OS;
echo "<br/>";
// 输出圆周率
echo M_PI;
echo "<br/>";
// 输出科学常数e
echo M_E;
echo "<br/>";
// 输出以2为底e的对数
echo M_LOG2E;
echo "<br/>";
// 输出以10为底e的对数
echo M_LOG10E;
echo "<br/>";
// 输出2的自然对数（常数e为底数的对数叫做自然对数）
echo M_LN2;
echo "<br/>";
// 输出10的自然对数
echo M_LN10;
echo "<br/>";
//（3）、php类常量
class MyClass{
//类常量，用constant修饰，不带
const cons1 = 'constant value';
function showCons1() {
echo self::cons1."<br/>";
}
}
//常量的表示形式： 类名::常量名
echo "<br/>类常量值为：".MyClass::cons1."<br/>";
?>
</span>1. If two identical constants are defined, which one will work, the former or the latter?
The former works because a constant cannot be redefined or undefined once it is defined.
2. What are the differences between constants and variables?
1) There is no $ sign before constants;
2) Constants can only be defined through define(), not assignment statements;
3) Constants can be defined and accessed anywhere, while variables have global and local;
4) Once a constant is defined, it cannot be redefined or undefined, while a variable is redefined through assignment;
5) The value of a constant can only be scalar data, and the database type of the variable There are 8 primitive data types.
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