How to define and use constants in PHP

1. Custom Constants

* Must be defined using the define () function
* After definition, the value cannot be changed.
* The constant name is used directly. You cannot add $ s in front of a variable.
For example: define ("PI", 3.14); defines a constant
$ Area = PI * R; Calculate the area of the circle
Define ("URL", "http://www.jb51.net ");
Echo "my website is:". URL;

2 system constants:

FILE: php program FILE name
LINE: Number of PHP Program Files
PHP_VERSION: version number of the current parser
PHP_ OS: name of the operating system for executing the current PHP version
You can use it directly. For example, to view the operating system name of the current PHP version, you can write it as echo PHP_ OS.

Define and use a class constant in php

Php Constants

We can define constants in the class. The constant value remains unchanged. You do not need to use the $ symbol when defining and using constants.

The constant value must be a fixed value and cannot be a variable, class attribute, or result of other operations (such as function call.

Its also possible for interfaces to have constants. Look at the interface documentation for examples. constants can also be defined in the interface. Please refer to the interface documentation for more examples.

After PHP5.3.0, we can use a variable to dynamically call the class. However, the value of this variable cannot be self, parent, or static.

Define and use a class constant
Copy codeThe Code is as follows:
<? Php
Class MyClass
{
Const constant = 'constant value ';
Function showConstant (){
Echo self: constant. "\ n ";
}
}

Echo MyClass: constant. "\ n ";

$ Classname = "MyClass ";
Echo $ classname: constant. "\ n"; // After PHP 5.3.0

$ Class = new MyClass ();
$ Class-> showConstant ();

Echo $ class: constant. "\ n"; // After PHP 5.3.0
?>

Example #2 static data Example

Copy codeThe Code is as follows:
<? Php
Class foo {
// After PHP 5.3.0
Const bar = <'eot'
Bar
EOT;
}
?>