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What is the difference between constant variables in PHP? How to determine whether a constant exists?
What is the difference between constant variables in PHP? How to determine whether a constant exists?
We have learned so much about PHP, what is the difference between constant variables in PHP today? How to judge whether a constant exists? I wonder if you have fully mastered it. If not, then follow this article to continue learning
Related recommendations:One minute to figure out what is predefined in PHP variable? What is the $_POST variable?
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
Recommended learning: "PHP Video Tutorial"
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