Constant and define() Function

PHP Tutorials - Herong's Tutorial Examples

∟Constant and define() Function

This section describes what is a constant, define() function defines a constant name to a value, constant value can retrieved by the name directly or by the constant() function, any string can be used as constant name.

What Is a Constant? A constant is an identifier for a data value. Once defined, the data value identified by a constant can not be changed. PHP supports constants with following basic rules:

1. A constant must be defined with the define(name, value) function. For example, define("PI", 3.14159); defines a constant named as PI to a value of 3.14159.

2. To retrieve the value defined in a constant, you can use the constant name directly in any expression. For example, define("PI", 3.14159); $area = PI * $radius * $radius; uses the value define in PI to calculate the area.

3. You are allowed to use any string for the constant name. For example, define("LONG PI", 3.14159265359); defines a constant named as 'LONG PI' to a value of 3.14159265359. But using special characters in constant names are not recommended.

4. If a constant name contains special characters, you need to use the constant(name) function to retrieve the value defined in the constant. For example, define("LONG PI", 3.14159265359); $area = constant("LONG PI") * $radius * $radius; uses the value defined in "LONG PI" to calculate the area.

5. If you want to know if there is a constant defined for a given constant name, you can use the define(name) function. For example, define("LONG PI", 3.14159265359); defined("LONG PI"); returns TRUE. And defined("LONG RI") returns FALSE.

6. Constants can be defined with only value of scalar data types: boolean, integer, float, and string.

7. The PHP engine provides a number of pre-defined (built-in) constants. For example, __FILE__, __LINE__, PHP_OS, PHP_VERSION, PHP_INT_MAX, DIRECTORY_SEPARATOR, ...

Now let's look at my tutorial example below to confirm some rules mentioned above:

<?php
#  ConstantTest.php
#- Copyright 2003 (c) HerongYang.com. All Rights Reserved.
#
   print "\n PI is not defined as a constant:\n";
   print "    PI: "; var_dump(PI);

   define("PI", 3.14159);
   print "\n PI is defined to a constant now:\n";
   print "    PI: "; var_dump(PI);

   define("LONG PI", 3.14159265359);
   print "\n LONG PI is defined to a constant now:\n";
   print "    LONG PI: "; var_dump(constant("LONG PI"));

   print "\n LONG RI is not defined as a constant:\n";
   print "    LONG PI: "; var_dump(defined("LONG PI"));
   print "    LONG RI: "; var_dump(defined("LONG RI"));

   print "\n Trying to define DAYS to an array:\n";
   define("DAYS", array("Jan"=>31,"Feb"=>28));
   print "    DAYS: "; var_dump(DAYS);

   print "\n PHP built-in constants:\n";
   print "    __FILE__: "; var_dump(__FILE__);
   print "    __LINE__: "; var_dump(__LINE__);
   print "    PHP_OS: "; var_dump(PHP_OS);
   print "    PHP_VERSION: "; var_dump(PHP_VERSION);
   print "    PHP_INT_MAX: "; var_dump(PHP_INT_MAX);
   print "    DIRECTORY_SEPARATOR: "; var_dump(DIRECTORY_SEPARATOR);
?>

If you run this sample script, you should get:

herong> \php\php ConstantTest.php

 PI is not defined as a constant:
    PI: string(2) "PI"

 PI is defined to a constant now:
    PI: float(3.14159)

 LONG PI is defined to a constant now:
    LONG PI: float(3.14159265359)

 LONG RI is not defined as a constant:
    LONG PI: bool(true)
    LONG RI: bool(false)

 Trying to define DAYS to an array:

Warning: Constants may only evaluate to scalar values in
C:\herong\ConstantTest.php on line 20
    DAYS: string(4) "DAYS"

 PHP built-in constants:
    __FILE__: string(43) "C:\herong\ConstantTest.php"
    __LINE__: int(25)
    PHP_OS: string(5) "WINNT"
    PHP_VERSION: string(5) "5.0.4"
    PHP_INT_MAX: int(2147483647)
    DIRECTORY_SEPARATOR: string(1) "\"