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

Note


  • 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


	      	  <?php
					define("MESSAGE", "Welcome to studentsempire.com!");
					echo MESSAGE;
	      	  ?>
	      

output


	      	  Welcome to studentsempire.com!	      


Example 2


	      	  <?php
					define("MESSAGE", "Welcome to studentsempire.com!", true);
					echo message;
	      	  ?>
	      

output


	      	  Welcome to studentsempire.com!	      


Constants are Global


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


Example 3


	      	  <?php
					define("MESS", "Welcome to studentsempire.com!");

					function myTest() {
					    echo MESS;
					}
					 
					myTest();
	      	  ?>
	      

output


	      	  Welcome to studentsempire.com!	      


Example 4


	      	  <?php
					define("aValue", 8); 
  					print aValue; 
	      	  ?>
	      

output


	      	  8	      


Example 5


	      	  <?php
					define("SecondsPerDay", 86400, true); 
					print SecondsPerDay; 
					print SECONDSperDAY; 
	      	  ?>
	      

output


	      	  8640086400	      


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.


	      	  <?php
					define("SecondsPerDay", 86400, true); 
					if (defined("Secondsperday")) { 
					    // etc 
					} 
	      	  ?>
	      


Example 7


constant() returns the value of a constant.


	      	  <?php
					define("SecondsPerDay", 86400, true); 
					$somevar = "Secondsperday"; 
					print constant($somevar); 
	      	  ?>
	      


Example 8


	      	  <?php
			    $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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