PHP Constants

PHP Constants are the identifiers that cannot be changed. Learn how to create a PHP Constant, and know more about pre-defined constants & Magic constants with examples:

In this tutorial, you will learn what a PHP constant is, how to create PHP constants (using the PHP define() function and the const keyword), predefined constants, magic constants, differences between constants and variables, and frequently asked questions (FAQs).

Please note that we have used PHP version 7 in all examples.

Let’s begin!

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Table of Contents:

PHP Constants: In-Depth Guide
How to Create a PHP Constant
Core Predefined Constants
Magic Constants
Differences: Constants Vs Variables
Frequently Asked Questions
Conclusion

PHP Constants: In-Depth Guide

A PHP constant is a name for a simple value. Unlike variables, we cannot change constants while running the script.

The characteristics of constants are:

A constant name starts with a letter or _ (the underscore character).
Unlike variables, no leading $ sign.
By default, constant names are case sensitive (we can make them case insensitive while defining).
The scope is global.

How to Create a PHP Constant

In PHP, there are two ways to define a constant, as shown in the list below:

using the define() function
using the const keyword

#1) Using the PHP define() Function

The syntax is as follows:

define(name, value, case-insensitive)

The above syntax has three parameters: name (required parameter), value (required parameter), and case-insensitivity. The Default value of case-insensitivity is false and is only required if you create a case-insensitive constant name.

A few valid constant names are:

define(“FOO”, “some text”);
define(“FOO2”, “some text”);
define(“FOO_BAR”, “some text”);

An invalid constant name includes:

define(“2FOO”, “some text”);

It is invalid, as the name cannot start with a number.

Let’s look at some examples.

You can practice these examples by running the following programming codes.

Example 1:

<?php
  
define("WELCOME", "Welcome to My World!");
 
echo WELCOME;
 
?>

Output:

Welcome to My World!

Example 2: An array constant

Note: We will cover arrays in a later tutorial.

<?php
  
define("colors", ["Red","Yellow","Green"]);
 
echo colors[0];
 
?>

Output:

Red

#2) Using the const Keyword

The syntax is as follows:

const NAME = value;

In the above syntax, NAME is not case-sensitive but is usually written in uppercase.

Let’s look at some examples.

You can practice these examples by running the following programming code.

Example 1:

<?php
  
const MY_NUM = 7;
 
echo MY_NUM;
 
?>

Output:

Example 2:

<?php
  
class MyPet {
  const NAME = "Tommy";
}
 
echo MyPet::NAME;
 
?>

Output:

Tommy

Core Predefined Constants

They are defined by the PHP core. There are 58 predefined constants, as shown in the list below:

PHP_VERSION
PHP_MAJOR_VERSION
PHP_MINOR_VERSION
PHP_RELEASE_VERSION
PHP_VERSION_ID
PHP_EXTRA_VERSION
PHP_ZTS
PHP_DEBUG
PHP_MAXPATHLEN
PHP_OS
PHP_OS_FAMILY
PHP_SAPI
PHP_EOL
PHP_INT_MAX
PHP_INT_MIN
PHP_INT_SIZE
PHP_FLOAT_DIG
PHP_FLOAT_EPSILON
PHP_FLOAT_MIN
PHP_FLOAT_MAX
DEFAULT_INCLUDE_PATH
PEAR_INSTALL_DIR
PEAR_EXTENSION_DIR
PHP_EXTENSION_DIR
PHP_PREFIX
PHP_BINDIR
PHP_BINARY
PHP_MANDIR
PHP_LIBDIR
PHP_DATADIR
PHP_SYSCONFDIR
PHP_LOCALSTATEDIR
PHP_CONFIG_FILE_PATH
PHP_CONFIG_FILE_SCAN_DIR
PHP_SHLIB_SUFFIX
PHP_FD_SETSIZE
E_ERROR
E_WARNING
E_PARSE
E_NOTICE
E_CORE_ERROR
E_CORE_WARNING
E_COMPILE_ERROR
E_COMPILE_WARNING
E_USER_ERROR
E_USER_WARNING
E_USER_NOTICE
E_RECOVERABLE_ERROR
E_DEPRECATED
E_USER_DEPRECATED
E_ALL
E_STRICT
__COMPILER_HALT_OFFSET__
true
false
null
PHP_WINDOWS_EVENT_CTRL_C
PHP_WINDOWS_EVENT_CTRL_BREAK

The list below shows more details about selected predefined constants.

PHP_VERSION
- Description – The current version of PHP within the format of major.minor.release[extra].
- Output data type – string
PHP_MAJOR_VERSION
- Description – The current major version of PHP.
- Output data type – int
- Ex: 7 from version 7.2.0
PHP_MINOR_VERSION
- Description – The current minor version of PHP.
- Output data type – int
- Ex: 2 from version 7.2.0
PHP_RELEASE_VERSION
- Description – The current release version of PHP.
- Output data type – int
- Ex: 0 from version 7.2.0
PHP_VERSION_ID
- Description – The current version of PHP. It is useful when comparing versions.
- Output data type – int
- Ex: 70200 from version 7.2.0
PHP_EXTRA_VERSION
- Description – The current extra version of PHP. Useful to specify a package version.
- Output data type – string
PHP_DEBUG
- Description – Assist in debugging PHP code.
- Output data type – int
PHP_MAXPATHLEN
- Description – The maximum length of file names (including path) is supported by this PHP build.
- Output data type – int
PHP_OS
- Description – The operating system (OS) PHP was built for.
- Output data type – string
PHP_OS_FAMILY
- Description – The OS family PHP was built for.
- Output data type – string
- Available since PHP 7.2.0
PHP_INT_MAX
- Description – The largest integer supported during this PHP build.
- Output data type – int
PHP_INT_MIN
- Description – The smallest integer supported during this PHP build.
- Output data type – int
PHP_INT_SIZE
- Description – The size of an integer in bytes during this PHP build.
- Output data type – int
PHP_FLOAT_MIN
- Description – Smallest representable positive floating-point number.
- Output data type – float
- Available since PHP 7.2.0
PHP_FLOAT_MAX
- Description – The largest representable floating-point number.
- Output data type – float
- Available since PHP 7.2.0
E_ERROR
- Description – Fatal run-time errors.
- Output data type – int
E_WARNING
- Description – Run-time warnings (non-fatal errors)
- Output data type – int
E_PARSE
- Description – Compile-time parse errors.
- Output data type – int
E_NOTICE
- Description – Run-time notices.
- Output data type – int
E_ALL
- Description – All errors and warnings (as supported).
- Output data type – int

Magic Constants

PHP provides a special set of predefined constants called magic constants. They can change depending on the place where they are used. They start with two underscores and also end with two underscores. Furthermore, magic constants are case-insensitive.

The following table shows the magic constants in PHP:

	Name	Description
1	__LINE__	The line number of the current file.
2	__FILE__	The full path of the file and the file name with symbolic links resolved. If it uses inside an include, the included file name will return.
3	__DIR__	The directory of the file. If it uses inside an include, the included file directory will return. It is equivalent to dirname(__FILE__). If it is not the root directory, it will not contain a trailing slash.
4	__FUNCTION__	The name of the function.
5	__CLASS__	The name of the class. It includes the namespace it was declared in (Ex: Foo\Bar). If it is used in a trait method, __CLASS__ is the class name the trait is used in.
6	__TRAIT__	The name of the trait. It includes the namespace it was declared in (Ex: Foo\Bar).
7	__METHOD__	The name of the class method.
8	__NAMESPACE__	The name of the current namespace.
9	ClassName::class	The name of the fully qualified class.

Differences: Constants Vs Variables

Constants and variables have similarities, but there are some significant differences, too.

	Constants	Variables
1	A name or an identifier for a value.	A name or symbol to store a value.
2	Syntax: define(name, value, case-insensitive)	Syntax: Leading $ sign followed by the name of the variable (example: $name)
3	The value cannot be changed once assigned.	Can be reassigned a value.
4	No leading $ sign is required.	Must have a leading $ sign to be executed.
5	By default, constants are global (universally global).	Variables can be local or global.
6	Useful for storing information that does not change.	Useful for storing information that might change.
7	Example:	Example:
	define(‘TEST’, ‘Software Testing Help’)	$test = ‘Software Testing Help’

Frequently Asked Questions

1. What are PHP constants?

Constants are names used for simple values. Furthermore, they cannot be changed while running the script.

2. How is a PHP constant created?

There are two methods to create a constant. They are using the PHP define() function and using the const keyword.

3. Give an example of a PHP constant.

Please refer to the example below:

const TEST = ‘Software Testing Help’;

(OR)

define(‘TEST’, ‘Software Testing Help’)

4. Can we redefine a constant in PHP?

No, unlike variables, we cannot redefine a constant once it is defined.

5. Show in an example how a function can access a constant that is defined outside the function.

A function can easily access a constant as constants have a global scope. Please refer to the example below:

<?php
define(“GREETING”, “Have a nice day!”);

function myExample() {
echo GREETING;
}

myExample();
?>

It will echo the following output:

Have a nice day!

6. What is the main difference between const and define in PHP?

The main difference is that const defines a constant at compile-time, whereas define() defines a constant at run-time.

Conclusion

A PHP constant is a name for a simple value and cannot be changed later on. The PHP define() function or the const keyword is used to create a constant.

Unlike variables, constants are accessible from anywhere in the script. Furthermore, there are predefined constants as well as magic constants in PHP.

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PHP Constants: In-Depth Guide

How to Create a PHP Constant

Core Predefined Constants

Magic Constants

Differences: Constants Vs Variables

Frequently Asked Questions

Conclusion

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