Programming Tutorials

Constants in PHP

By: Andi, Stig and Derick in PHP Tutorials on 2008-11-22  

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])


  • "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");

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