Constants in PHP

Constants, like variables, store a certain value, only unlike variables, the value of constants can be set only once, and then we can no longer change it. Constants are usually defined to store values ​​that must remain the same throughout the duration of the script.

const operator

The const operator is used to define a constant, and the dollar sign $ (unlike variables) is not used in the name of the constant.

<?php
const PI = 3.14;
echo PI;
?>

Usually, the names of constants use capital characters, but this is a convention.

After defining a constant, we can use it just like a normal variable.

PHP allows you to set constant values ​​based on evaluated expressions:

 
<?php
const PI = 2.1415 + 1;
echo PI; // 3.1415
?>

The only exception is that we cannot change the value of the constant. That is, an expression PI = 3.1415; that should change the value of a constant will not work.

define function

You can also use the define() function to define a constant, which has the following form:

define(string $name, string $value)

The parameter $namepasses the name of the constant, and the parameter $value- its value. The value of a constant can represent a type int, float, string, bool, null, or array.

For example, let’s define a numeric constant:

<?php
define("NUMBER", 22);
echo NUMBER; // 22
?>

Magic Constants

In addition to the constants created by the programmer, PHP has several more so-called “magic” constants that are in the default language:

• __FILE__ : stores the full path and name of the current file
• __LINE__ : stores the current line number that the interpreter is processing
• __DIR__ : stores the directory of the current file
• __FUNCTION__ : name of the function being processed
• __CLASS__ : current class name
• __TRAIT__ : name of the current trait
• __METHOD__: name of the method being processed
• __NAMESPACE__ : name of the current namespace
• ::class : full name of the current class

For example, let’s print the current executable line and file name:

<?php
echo "String " . __LINE__ . " in file " . __FILE__;
?>

Checking for the Existence of a Constant

To check if a constant is defined, we can use the bool-defined (string $name) function. If a constant $ name is defined, then the function will return a value true:

<?php
const PI = 3.14;
if (!defined("PI"))
    define("PI", 3.14);
else
    echo "PI constant already defined";
?>