Constants

• A constant is a identifier or name for a simple value. As the name suggests, that value cannot change during the execution of the script.
• A constant identifier is case-sensitive by default.
• By convention constant identifiers are always uppercase.
• The scope of a constant is global -- you can access it anywhere in your script without regard to scope.
• You define a constant by using the define() function.
• Once a constant is defined, it can never be changed or undefined.
• Only scalar data (boolean, integer, float and string, not arrays) can be contained in constants.

  <?php
  define("PI", 3.14159 );
  $radius = 6378.14 ; // mean equitorial radius of the earth in kilometers
  $volume = ( 4 / 3 ) * PI * $radius^3 ;  // Calculate the approximate volume of the earth
  echo "Radius: $radius km" ;
  echo "Volume: $volume cubic kilometers" ;
  ?>
  

  Since names for constants can be made from any letters (a to z and A to Z) and from ASCII characters 128 through 255, you can name constants as follows (if you wanted to):

  define("¢", "Cents") ; // ¢ = ASCII 155
  define("£", "Pounds Sterling") ; // £ = ASCII 156
  echo "Do you want that in " . £ . " or in Dollars and " . ¢ . "?" ;