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");