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PHP Defining Constants

Last Updated : 23 Feb, 2023
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In a production-level code, it is very important to keep the 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 that 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 automatic 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.

Note: From PHP 8.0, the constants defined with the help of the define() function may be case-insensitive.

Below are some examples to illustrate the working of the define() function in PHP.

Example 1: Below program illustrates defining case-insensitive constants.


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


Hello Geeks!  // Case Insensitive thus value is echoed
Hello Geeks!

Example 2: Below program illustrates defining case-sensitive constants.


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


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

The PHP compiler will also throw a warning for the 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) that generally don’t change over time.
  • Constants are irrespective of the scope and always populate the global scope.
  • define() method is used to define constants.
  • The defined() method is used to check if a constant is defined.
  • The constant() method is used to return the value of a constant and NULL if not the constant is not defined. 

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