What is the difference between constant variables in PHP? How to determine whether a constant exists?

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The difference between constant variables:

• Different definition forms:

• Different forms of use: constants do not require the $ symbol

• Different degrees of variability: the value of a constant cannot be changed, and the constant cannot be destroyed

• Different scopes: constants have super-global scope (can be used directly inside and outside functions)
  

• The available types are different: constants can only store scalar types (integers, floating point numbers, characters String, Boolean)

Judge whether the constant exists

Use the defined0 function:

• If it exists: the return result is true,

• If it does not exist: the return result is false|

if( defined("PI") ){
echo "<br />常量PI已经存在";》//通常 此时就可以去使用它!
}
else{
echo "<br />常量PI不存在";//通常，判断不存在，是为了来定义它!
define("PI", 3.14);//然后去使用
$s3=PI*55;
echo "<br />面积为: $s3";
if( defined("G") ){
echo "<br />常量G已经存在"; //通常此时就可以去使用它!
}
else{
echo "<br />常量G不存在";//通常，判断不存在，是为了来定义它!
define("G", 9.8);% //然后去使用， G是”重力加速度"
}
$s4=G* 6 ; //6为时间(秒)，这里是计算得到速度
echo "<br />速度为: $s4";

Use An undefined constant:

First look at the two comparison codes:

echo“v1 的值为”
$v1;
//注意，该变量v1未定义过
echo“C1 的值为”，
C1;
//注意，该常量Cl未定义过

In php, when using an undefined constant , the system will directly use the constant as a "valued" constant, and its value is the name of the constant - although an error will be reported!

Predefined constants:

are some predefined constants in the system, there are about hundreds of them

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