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If you look up the word constant in a dictionary it will probably tell you that the word is used to describe something that is non-changing and non-variable, and this is exactly the purpose of constants in PHP. A PHP constant is the opposite of a variable in that once it has been defined it cannot be changed.

Constants are particularly useful for defining a value that you frequently need to refer to that does not ever change. For example, you might define a constant called INCHES_PER_YARD that contains the number of inches in a yard. Since this is a value that typically doesn't from one day to the next it makes sense to define it as a constant. Conversely, a value that is likely to change, such as the Dollar to Yen exchange rate is best defined as a variable.

PHP constants are said to have global scope. This basically means that once you have defined a constant it is accessible from any function or object in your script.

In addition, PHP provides a number of built-in constants that are available for use to make life easier for the PHP developer.

	Contents 1 Defining a PHP Constant 2 Checking if a PHP Constant is Defined 3 Using a Variable as a Constant Name 4 Predefined PHP Constants 4.1 PHP Script and Environment Related Constants 4.2 PHP Mathematical Constants

Defining a PHP Constant

Rather than using the assignment operator as we do when defining variables, constants are created using the define() function. Constants also lack the $ prefix of variable names.

The define function takes two arguments, the first being the name you wish to assign to the constant, and the second the value to assign to that name.

Constant names are case sensitive. Although it is not a requirement, convention carried over from other programming languages suggests that constants should be named in all upper case characters. The following example demonstrates the use of the define() function to specify a constant:

<?php
define('INCHES_PER_YARD', 36);
?>

Once defined the constant value can be accessed at any time just by referring to the name. For example, if we define a string constant as follows:

<?php
define('WELCOME_MESSAGE', "Welcome to my World");
?>

we can subsequently access that value anywhere in our script. For example, to display the value:

echo WELCOME_MESSAGE;

Checking if a PHP Constant is Defined

It can often be useful to find out if a constant is actually defined. This can be achieved using the PHP defined() function. The defined() function takes the name of the constant to be checked as an argument and returns a value of true or false to indicate whether that constant exists.

For example, let's assume we wish to find out if a constant named MY_CONSTANT is defined. We can simply call the defined() function passing through the name, and then test the result using an if .. else statement (see PHP Flow Control and Looping for more details on using if .. else):

<?php
define ('MY_CONSTANT',  36);

if (defined('MY_CONSTANT'))
{
    echo "Constant is defined";
}
else
{
    echo "Constant is not defined";
}
?>

Using a Variable as a Constant Name

It is not always the case that you want to hard-code a constant name into a script at the point you wish to access it. For example, you may have a general purpose script that you wish to perform tasks on any number of different constants, not just one that you happen to have typed in the name for. The best way to resolve this issue is store the name of the constant in a variable. How then, would you access the value assigned to that constant? The answer is to use the PHP constant() function. The constant() function takes the name of the constant as an argument and returns the value of the constant which matches that name.

The key point to understand here as that the argument passed through to constant() can be a string variable set to the name of the constant.

As always an example helps a great deal in understanding a concept. In the script below we define a constant called MY_CONSTANT. Next, we create a string variable called constantName and assign it a value of MY_CONSTANT (i.e. a string that matches the constant name). We can then use this new variable as the argument to constant() to obtain the constant value of MY_CONSTANT:

<?php
define ('MY_CONSTANT', "This is a constant string.");
$constantName = 'MY_CONSTANT';

if (defined ( $constantName ))
{
      echo constant($constantName);
}
else
{
      echo "$constantName constant is not defined.";
}
?>

The above script will display the constant value if it exists by using the value of the $constantName variable constant name key.

Predefined PHP Constants

As we mentioned briefly at the start of this chapter, PHP provides a number of built-in constants that can be of significant use to the PHP web developer. In this section we will look at some of the more useful constants available.

PHP Script and Environment Related Constants

The following constants provide information about the script which is currently executing, and also the environment in which the PHP web server module is running. These are of particular use when debugging scripts:

Constant Name	Description
__LINE__	Contains the number of the line in the current PHP file (or include file) which is being currently being executed by the PHP pre-processor.
__FILE__	Contains the name of the file or include which contains the currently executing line of PHP code.
__FUNCTION__	Contains the name of the PHP function which is currently executing
__CLASS__	Contains the class which is currently in use
__METHOD__	Contains the name of the method in the current class which is currently executing
PHP_VERSION	Contains the version of PHP that is executing the script
PHP_OS	Contains of the name of the Operating System hosting the PHP Pre-processor
PHP_EOL	Contains the Newline character for the host OS (differs between UNIX/Linux and Windows for example)
DEFAULT_INCLUDE_PATH	The default path where PHP looks for include files

PHP Mathematical Constants

PHP provides a number of useful mathematical constants that can be used to save both programming time when writing a script, and processing time when performing calculations in a script. The following table provides a list of the mathematical constants available in PHP:

Constant	Description
M_E	Value of e
M_EULER	Value of Euler's constant
M_LNPI	The natural logarithm of PI
M_LN2	The natural logarithm of 2
M_LN10	The natural logarithm of 10
M_LOG2E	Value of base-2 logarithm of E
M_LOG10E	The base-10 logarithm of E
M_PI	The value of PI
M_PI_2	The value of PI/2
M_PI_4	The value of PI/4
M_1_PI	The value of 1/PI
M_2_PI	The value of 2/PI
M_SQRTPI	The square root of PI
M_2_SQRTPI	The value 2/square root of PI
M_SQRT2	The square root of 2
M_SQRT3	The square root of 3
M_SQRT1_2	The square root of 1/2

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