Do you know what php constants are? How to check whether it has been declared?

In the previous article "Do you know how to set values ​​for PHP variables", we learned what variables are, methods of variable declaration and variable assignment. This time we will learn about constants, how they are declared, and how to detect whether a constant has been declared.

Before understanding constant declaration and detecting whether a constant exists, we need to first understand What is a constant.

The so-called constant is a quantity that cannot be changed. Once defined, it cannot be modified or undefined; it is usually used to store data that does not change and is not expected to change.

Variables are divided into local and global, but constants are different. They run through the entire script, and the scope is automatically global.

And variables can store many types of data, while constants can only store data of integer, floating point, string, Boolean, and array types (starting with PHP7).

Variable declaration uses the form of "$variable name=value;", but constants cannot be declared. The declaration of constants requires the use of the define() function or the const keyword.

Let’s take a look at the following example:

<?php
header("Content-Type: text/html;charset=utf-8");    //设置字符编码
define("CONSTANT", "Hello world!");
const name = &#39;李华&#39;;
echo CONSTANT.&#39;<br>&#39;;
echo name;
?>

In the above code, we use the define() function to declare a constant "CONSTANT" with a value of " Hello world!"; uses the const keyword to declare a constant "name" with the value "李华". Therefore, when echo is used to output these two constants (there is no dollar sign $ in front of the constant), the output will be:

Hello world!
李华

Now that we understand how to declare constants, let’s take a lookMethod to detect whether a constant has been declared (whether it exists) .

Let’s take a look at an example:

<?php
header("Content-Type: text/html;charset=utf-8");    //设置字符编码
define("CONSTANT", "Hello world!");

if(defined(&#39;CONSTANT&#39;)){
	echo "常量 CONSTANT 是存在的，值为： ".CONSTANT;
}else{
	echo "常量 CONSTANT 是不存在的 ";
}

const name = &#39;李华&#39;;
if(defined(&#39;name&#39;)){
	echo "<br>常量 name 是存在的，值为： ".name;
}else{
	echo "<br>常量 name 是不存在的 ";
}
?>

The output result is:

It can be seen that we Just use the defined() function to check whether the constant exists. Let’s take a brief look at the function:

defined($name)The function only accepts one parameter $name (the name of the constant to be detected, because it is a string type, So it needs to be wrapped in quotation marks.) will return a bool type value. If the constant is defined, it returns true, if it is not defined, it returns false.

Okay, that’s all. If you want to know anything else, you can click this. → →php video tutorial

Finally, I recommend a free video tutorial on PHP arrays: Enter the world of PHP from 0, come and learn!

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