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PHP | Defining Constants
  • Difficulty Level : Basic
  • Last Updated : 03 Jan, 2018

In a production level code it is very important to keep information as either variables or constants rather than using them explicitly. A PHP constant is nothing but an identifier for a simple value that tends not to change over time(such as the domain name of a website eg. It is ideal to keep all the constants in a single PHP script so the maintenance is made easier. A valid constant name must start with an alphabet or underscore and requires no ‘$’. It is to be noted, constants are irrespective of their scope i.e constants are automatically of global scope.

In order to create a constant in PHP, we must use the define() method.


bool define(identifier, value, case-insensitivity)

Parameters: The function has two required parameters and one optional parameter.

  • identifier: Specifies the name to be assigned to the constant.
  • value: Specifies the value to be assigned to the constant.
  • case-insensitivity(Optional): Specifies whether the constant identifier should be case-insensitive. By default it is set to false i.e. case-sensitive.

Return Type: This method returns TRUE on success and FALSE on Failure.

Below are some examples to illustrate working of define() function:

  • Below program illustrates defining case-insensitive constants:

      // case-insensitive costants
      define("Constant","Hello Geeks!",TRUE);
      echo constant;
      echo Constant;

    Output :

    Hello Geeks!  // Case Insensitive thus value is echoed
    Hello Geeks!
  • Below program illustrates defining case-sensitive constants:

      // case-sensitive constant
      define("Constant","Hello Geeks!");
      echo constant;
      echo Constant;

    Output :

    constant   // Case Sensitive thus value not echoed
    Hello Geeks! 

    The PHP compiler will also throw a warning for above program along with the output as: “PHP Notice: Use of undefined constant constant- assumed ‘constant’ in line 5”.


  • Constants are identifiers that can be assigned values(string, boolean, array, integer, float or NULL) which generally don’t change overtime.
  • Constants are irrespective of scope and always populate the global scope.
  • define() method is used to define constants.
  • defined() method is used to check if a constant is defined.
  • constant() method is used to return the value of a constant and NULL if not the constant is not defined.

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