PHP: Constants in PHP

In PHP, you can define names, called constants, for simple values. As the name implies, you cannot change these constants once they represent a certain value. The names for constants have the same rules as PHP variables except that they don’t have the leading dollar sign. It is common practice in many programming languages - including PHP - to use uppercase letters for constant names, although you don’t have to. If you wish, which we do not recommend, you may define your constants as case-insensitive, thus not requiring code to use the correct casing when referring to your constants.

Tip: Only use case-sensitive constants both to be consistent with accepted coding standards and because it is unclear if case-insensitive constants will continued to be supported in future versions of PHP.

Unlike variables, constants, once defined, are globally accessible. You don’t have to (and can’t) redeclare them in each new function and PHP file.

To define a constant, use the following function:

define("CONSTANT_NAME", value [, case_sensitivity])

Where:

. "CONSTANT_NAME" is a string.

. value is any valid PHP expression excluding arrays and objects.

. case_sensitivity is a Boolean (true/false) and is optional. The default is true.

An example for a built-in constant is the Boolean value true, which is registered as case-insensitive.

Here’s a simple example for defining and using a constant:

define("MY_OK", 0);

define("MY_ERROR", 1);

...

if ($error_code == MY_ERROR) {

print("There was an error\n");

}

Constants in PHP

A constant is a name or an identifier for a simple value. A constant value cannot change during the execution of the script. By default a constant is case-sensitiv. By convention, constant identifiers are always uppercase. A constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. If you have defined a constant, it can never be changed or undefined.
To define a constant you have to use define() function and to retrieve the value of a constant, you have to simply specifying its name. Unlike with variables, you do not need to have a constant with a $. You can also use the function constant() to read a constant's value if you wish to obtain the constant's name dynamically.

constant() function:

As indicated by the name, this function will return the value of the constant.
This is useful when you want to retrieve value of a constant, but you do not know its name, i.e. It is stored in a variable or returned by a function.

constant() example:

<?php define("MINSIZE", 50); echo MINSIZE; echo constant("MINSIZE"); // same thing as the previous line ?>

Only scalar data (boolean, integer, float and string) can be contained in constants.

Differences between constants and variables are:

• There is no need to write a dollar sign ($) before a constant, where as in Variable one has to write a dollar sign.
• Constants cannot be defined by simple assignment, they may only be defined using the define() function.
• Constants may be defined and accessed anywhere without regard to variable scoping rules.
• Once the Constants have been set, may not be redefined or undefined.

Valid and invalid constant names:

// Valid constant names define("ONE", "first thing"); define("TWO2", "second thing"); define("THREE_3", "third thing") // Invalid constant names define("2TWO", "second thing"); define("__THREE__", "third value");

PHP Magic constants:

PHP provides a large number of predefined constants to any script which it runs.
There are five magical constants that change depending on where they are used. For example, the value of __LINE__ depends on the line that it's used on in your script. These special constants are case-insensitive and are as follows:
A few "magical" PHP constants ate given below:

Name	Description
__LINE__	The current line number of the file.
__FILE__	The full path and filename of the file. If used inside an include,the name of the included file is returned. Since PHP 4.0.2, __FILE__ always contains an absolute path whereas in older versions it contained relative path under some circumstances.
__FUNCTION__	The function name. (Added in PHP 4.3.0) As of PHP 5 this constant returns the function name as it was declared (case-sensitive). In PHP 4 its value is always lowercased.
__CLASS__	The class name. (Added in PHP 4.3.0) As of PHP 5 this constant returns the class name as it was declared (case-sensitive). In PHP 4 its value is always lowercased.
__METHOD__	The class method name. (Added in PHP 5.0.0) The method name is returned as it was declared (case-sensitive).