calling a private array from another private array in php

I don't understand why doesn't this code work:

<?php
class Test {

    private $BIG = array(
        'a' => 'A',
        'b' => 'B',
        'c' => 'C'
    );

 开发者_高级运维   private $arr2 = array(
        $this->BIG['a'],
        $this->BIG['b'],
        'something'
    );

    public function getArr2(){
        return $this->arr2;
    }


}

$t = new Test();
print_r($t->getArr2());

?>

I get this error:

Parse error: syntax error, unexpected T_VARIABLE, expecting ')' in /home/web/te/test.php on line 11

You can't combine variables in a class member definition. You can only use native types and constants:

private $arr = array('a', 'b');
private $obj = new stdClass(); // error
private $obj = (object)array(); // error
private $num = 4;
private $a = 1;
private $b = 2;
private $c = $this->a + .... // error

If you want to combine or calculate, do that in __construct:

private $a = 1;
private $b = 2;
private $c;

function __construct() {
  $this->c = $this->a + $this->b;
}

You can't reference $this when declaring properties.

From the PHP Documentation:

[Property] declaration may include an initialization, but this initialization must be a constant value--that is, it must be able to be evaluated at compile time and must not depend on run-time information in order to be evaluated.

Therefore, do actions like that in the constructor:

class Test {

    private $BIG = array(
        'a' => 'A',
        'b' => 'B',
        'c' => 'C'
    );

    private $arr2;

    public function __construct()
    {
        $this->arr2 = array(
            $this->BIG['a'],
            $this->BIG['b'],
            'something'
        );
    }

    public function getArr2(){
        return $this->arr2;
    }
}