const PHP Parse error

Question

// define('DOCROOT', realpath(dirname(__DIR__)));
// good

const DOCROOT = realpath(dirname(__DIR__));
// PHP Parse error:  syntax error, unexpected '(', expecting ',' or ';' in

why error?

Answer 1

A constant can be defined two ways in PHP,

1. Using const keyword

   You can't assign the result of function, or even a variable to a constant in this way. the value of the constant (defined this way), must be a fixed value, like an integer, or a string.

2. Using define()

   in this way, you can assign any value or variable or result of any function to your constant.

Important note : define() Works OUTSIDE of a class definition.

Examples

$var = "String"; 
const CONSTANT = $string;                        //wrong
const CONSTANT = substr($var,2);                 //wrong
const CONSTANT = "A custom variable";            //correct
const CONSTANT = 2547;                           //correct
define("CONSTANT", "A custom variable");         //correct
define("CONSTANT", 2547);                        //correct
define("CONSTANT", $var);                        //correct
define("CONSTANT", str_replace("S","P", $var));  //correct

class Constants
{
  define('MIN_VALUE', '0.0');  //wrong - Works OUTSIDE of a class definition.
}

Answer 2

A class constant has to be a fixed value. Not the result of some function. Only global constants set by define may contain results of a function.

Global Constants: http://www.php.net/manual/en/language.constants.php

Class Constants: http://www.php.net/manual/en/language.oop5.constants.php

Sample for a global constant:

define("FOO",     "something");
echo FOO;

Sample for a class constant:

class Test {
    const FOO = "Hello";
}

echo Test::FOO;

Answer 3

Check this site: http://www.php.net/manual/en/language.constants.php

const definition must be inside the scope of one class.