PHP Constant

Constants are used to make sure a value does not change throughout the running of the script.

A constant is an identifier (name) for a simple value. The value cannot be changed during the script.

A valid constant name starts with a letter or underscore (no $ sign before the constant name).

Note: Unlike variables, constants are automatically global across the entire script.

  • Using define() function
  • Using const keyword

PHP constants follow the same PHP variable rules. For example, it can be started with letter or underscore only.

Conventionally, PHP constants should be defined in uppercase letters.

To define a constant, use the define() function, and include name of the constant, followed by the value for the constant, as shown here:

 define(name, value, case-insensitive) 

  • name: Specifies the name of the constant
  • value: Specifies the value of the constant
  • case-insensitive: Specifies whether the constant name should be case-insensitive. Default is false


  • Constants may only contain scalar values such as Boolean, integer, float, and string (not values such as arrays and objects).
  • Constants can be used from anywhere in your PHP program without regard to variable scope.
  • Constants are case-sensitive.

Example 1

					define("MESSAGE", "Welcome to!");
					echo MESSAGE;


	      	  Welcome to!	      

Example 2

					define("MESSAGE", "Welcome to!", true);
					echo message;


	      	  Welcome to!	      

Constants are Global

Constants are automatically global and can be used across the entire script.

Example 3

					define("MESS", "Welcome to!");

					function myTest() {
					    echo MESS;


	      	  Welcome to!	      

Example 4

					define("aValue", 8); 
  					print aValue; 



Example 5

					define("SecondsPerDay", 86400, true); 
					print SecondsPerDay; 
					print SECONDSperDAY; 



Example 6

The defined() function is basically the constant equivalent of isset(), as it returns true if the constant string you pass to it has been defined.

					define("SecondsPerDay", 86400, true); 
					if (defined("Secondsperday")) { 
					    // etc 

Example 7

constant() returns the value of a constant.

					define("SecondsPerDay", 86400, true); 
					$somevar = "Secondsperday"; 
					print constant($somevar); 

Example 8

			    $radius = 4; 
     	        $diameter = $radius * 2; 
			    $circumference = M_PI * $diameter; 
			    $area = M_PI * pow( $radius, 2 ); 

			    echo "A radius of " . $radius . " \n "; 
			    echo "A diameter of " . $diameter . " \n "; 
			    echo "A circumference of " . $circumference . " \n "; 
			    echo "An area of " . $area . " \n ";    

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