oop - Accessing PHP Class Constants

Question

The PHP manual says

Like static members, constant values can not be accessed from an instance of the object.

which explains why you can't do this

$this->inst = new Classname();
echo $this->inst::someconstant;

but then why does this work?

$this->inst = new Classname();
$inst = $this->inst;
echo $inst::someconstant;

Answer 1

From the PHP interactive shell:

php > class Foo { const A = 'a!'; }
php > class Bar { public $foo; }
php > $f = new Foo;
php > $b = new Bar;
php > $b->foo = $f;
php > echo $b->foo::A;
PHP Parse error:  syntax error, unexpected T_PAAMAYIM_NEKUDOTAYIM, expecting ',' or ';' in php shell code on line 1

The reason that the former syntax fails is that the PHP parser doesn't know how to resolve the double-colon after the property reference. Whether or not this is intentional is unknown.

The latter syntax works because it's not going through the property directly, but through a local variable, which the parser accepts as something it can work with:

php > $inst = $b->foo;
php > echo $inst::A;
a!

(Incidentally, this same restriction is in place for anonymous functions as properties. You can't call them directly using parens, you have to first assign them to another variable and then call them from there. This has been fixed in PHP's trunk, but I don't know if they also fixed the double colon syntax.)