PHP Tutorial - PHP Constants

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

Syntax

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( "MY_CONSTANT", "1" ); // MY_CONSTANT always has the string value "1"

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("aValue", 8); 
  print aValue; 
?>

The code above generates the following result.

Example 2

Pass in true as a third parameter to define() makes the constant case-insensitive:

<?PHP/*  w ww .ja  va2s  .  co  m*/
define("SecondsPerDay", 86400, true); 
print SecondsPerDay; 
print SECONDSperDAY; 
?>

The code above generates the following result.

Example 3

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.

For example:

<?PHP//from   ww  w .  j a v a2  s.c  om
define("SecondsPerDay", 86400, true); 
if (defined("Secondsperday")) { 
    // etc 
} 
?>

Example 4

constant() returns the value of a constant.

<?PHP/*from   w  w  w  .j  a v a  2  s.c  om*/
define("SecondsPerDay", 86400, true); 
$somevar = "Secondsperday"; 
print constant($somevar); 
?>

The code above generates the following result.

Example 5

Use Math constant to calculate circle area

<?php //from ww w  .  j  a  v  a 2 s  .co m
         $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 "; 
               
?>  

The code above generates the following result.