Can I set a constant in a class then access it outside in PHP?

Question

I am trying to initialize some values inside a class and save them in constant and access them outside, in different part of my code.

<?php

class Config {

  public static function initialize() {
    define('TEST',"This is a Constant");
  }

}

$config = Config::initialize();
// do something with the constants

Can I access it outside?

Answer 1

A Class constant uses the const keyword. You don't define them using the define function. Just like this:

class Config {
        const TEST = "This is a constant";
}

// then use it:
var_dump(Config::TEST);

In PHP, you cannot dynamically set the value of a constant, but you can get a similar behaviour with a public static variable. ie.

class Config2 {
    public static $test = null;
    public static function initialize()
    {
        self::$test = "This is not a constant";
    }
}

// Then use like
Config2::initialize();
var_dump(Config2::$test);

The downside is, there is nothing stopping other code from setting the value from outside the class. If you need protection against this, you should use a getter function approach. eg.

class Config3 {
    private static $_test = null;
    public static function initialize()
    {
        self::$_test = "This is not a constant, but can't be changed outside this class";
    }

    public static function getTest()
    {
        return self::$_test;
    }
}

// Then use like
Config3::initialize();
var_dump(Config3::getTest());

Answer 2

class Config {
  const TEST = "This is a Constant";

  static public $test = "This is a static property test."
  static protected $test2;

  public static function initialize() {
      self::$test2 = 'initialized';
      return self::$test2;
  }

  public static function getTest2()
  {
      return self::$test2;
  }
}

echo Config::TEST; // outputs This is a Constant
echo Config::$test; // outputs This is a static property test.
echo Config::initialize(); // outputs initialized;
echo Config::getTest2(); // outputs initialized;

If you need to dynamically set the value then you dont want to use a constant you want to use a static property. IF you only want the Config class to be able to manipulate the value of this property directly then use the private or protected keyword. If thats a non issue then you could use a public property.

Another and perhaps most robust approach depending on what you are trying to implement is to use constants to access static or instance specific properties of the class:

class Config
{
  const TEST = 0;
  const TEST2 = 1;

  protected static $conf = array();

  public static function initialize($testVal, $test2Val)
  {
    $conf[self::TEST] = $testVal;
    $conf[self::TEST2] = $test2Val;
  }

  public static function get($key)
  {
     if(isset(self::$conf[$key])
     {
       return self::$conf[$key];
     }
  }
}

Config::initialize('Test', 'Test 2');
echo Config::get(Config::TEST);
echo Config::get(Config::TEST2);

Answer 3

Not from your original code. But the following would work as a constant class variable.

class Config {
    const TEST = "This is a Class Constant";
...
}

Accessed from anywhere that includes the Config class declaration like:

echo Config::TEST;