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Revision as of 16:40, 29 May 2007
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 constatnt 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 change 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.
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 name are case sensitive. Althouth 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 (i.e 1 or 0) 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 Looping and Flow Control 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 happen to have typed in the name for. The best way to resolve thi sissue 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() fucntion. 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 matrches 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 the section we will look at some of the more common constants available.
PHP Script and Environment Related Constants
The following constants provide information about the script which currently being executed, and also the environment in which the PHP web server module is running. These are of particular use when debugging scripts:
Constant Name | Descrition |
---|---|
__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 current which is currently in use |
__METHOD__ | Contains the name of the method in the curerent 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 chacter 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 lists a subset of the constants available:
Constant | Description |
---|---|
M_E | Value of e (approx. 2.718) |
M_EULER | Value of Euler's constant (approx. 0.577) |
M_LNPI | The natural logarithm of PI (approx. 1.144) |
M_LN2 | The natural logarithm of 2 (approx. 0.693) |
M_LN10 | The natural logarithm of 10 (approx. 2.302) |
M_LOG2E | Value of base-2 logarithm of E (approx. 1.442) |
M_LOG10E | The base-10 logarithm of E (approx. 0.434) |
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 |