Detailed explanation of php custom/system/class constant code examples

Let’s look at a piece of code first:

 "; //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."
"; //（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 "
"; //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 "
"; //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 "
"; // 输出圆周率 echo M_PI; echo "
"; // 输出科学常数e echo M_E; echo "
"; // 输出以2为底e的对数 echo M_LOG2E; echo "
"; // 输出以10为底e的对数 echo M_LOG10E; echo "
"; // 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 "
"; // 输出10的自然对数 echo M_LN10; echo "
"; //（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."
"; } } //常量的表示形式： 类名::常量名 echo "
类常量值为：".MyClass::cons1."
"; ?> 

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 primitivedata types.

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