Why can't I do math in a class property declaration? [closed]

Answer 1

The documentation never states that it is a valid property declaration. It specifically states its an invalid example.

directly from your link

<?php
class SimpleClass
{
   // invalid property declarations:
   public $var1 = 'hello ' . 'world';
   public $var2 = <<<EOD
hello world
EOD;
   public $var3 = 1+2;
   public $var4 = self::myStaticMethod();
   public $var5 = $myVar;

   // valid property declarations:
   public $var6 = myConstant;
   public $var7 = array(true, false);

   // This is allowed only in PHP 5.3.0 and later.
   public $var8 = <<<'EOD'
hello world
EOD;
}
?>

this is just how the language syntax is defined for PHP.

Answer 2

Adding a default value for a property should be done inside the constructor. To add default values when declaring the property is just a non-OO thing that rolls in PHP to make live for developers easier, but -- as you can see -- it isn't supported very well yet.

The solution:

class Foo
{
    private $var;

    public function __construct()
    {
        $this->var = 2 + 3
    }
}

Answer 3

PHP does not support calculated values, even though both values technically can be known at compile time.

Answer 4

Just because you can't in PHP. You can't have evaluated property assignments aloong with declarations in PHP. Not sure why it was implemented this way.