New features in PHP version 5.4: How to use the const keyword to define constants

New features in PHP 5.4 version: How to use the const keyword to define constants

In PHP 5.4 version, a new keyword const is introduced to define constants more conveniently. In previous versions, we usually used the define() function to define constants, but in PHP 5.4, we can use a more concise way to define constants.

It is very simple to use the const keyword to define a constant. You only need to add the keyword const in front of the constant name. Here is an example:

<?php

const PI = 3.14;
const MY_NAME = "John Doe";

echo 'PI的值为：' . PI . '<br>';
echo '我的名字是：' . MY_NAME;

?>

In the above example, we defined two constants using the const keyword: PI and MY_NAME. The value of the constant PI is 3.14, and the value of the constant MY_NAME is "John Doe". When using these constants, we only need to use the constant name directly, and there is no need to use the $ symbol to represent it.

An important feature of using the const keyword to define constants is that they are determined at compile time. This means that the value of the constant is determined before the script is executed and cannot be modified. This is different from the way to define constants using the define() function, which dynamically defines constants at runtime.

Another thing to note is that when using the const keyword to define a constant, the $ sign does not need to be added to the constant name. This is different from the naming rules for variables.

In addition, constants defined using the const keyword can only be used in classes, not within functions. If you need to define constants inside a function, you still need to use the define() function.

The following is an example of using the const keyword to define constants in a class:

<?php

class MathConstants {
    const PI = 3.14;
    const E = 2.71;
}

echo 'PI的值为：' . MathConstants::PI . '<br>';
echo 'E的值为：' . MathConstants::E;

?>

In the above example, we define a class named MathConstants and use const in the class The keywords define two constants: PI and E. Use the class name and the double colon operator (::) to access these constants just like static properties of the class.

To summarize, a new keyword const was introduced in PHP 5.4 to define constants more conveniently. Constants defined through the const keyword are determined at compile time and cannot be modified. This method is more concise and intuitive than using the define() function to define constants, and is more in line with object-oriented programming ideas.

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