Why can you instantiate a class from a string variable but not a string in PHP?

Question

How come in PHP this works:

$myClass = 'App\MyClass';
$object = new $myClass;

But this results in an error:

$myClass = 'MyClass';
$object = new 'App\\'.$myClass;

In the second example, an unexpected T_CONSTANT_ENCAPSED_STRING is thrown.

As it turns out, the above example is due to operator precedence since new takes highest precedence, but...

Similarly, I can try instantiating with just a string as in:

$object = new 'App\MyClass';

And the same error is thrown. Why is this?

Answer 1

The parser's implementation is not generic, therefore in some cases the parser would hiccup over things the engine could in fact handle. There (still) isn't even an official formal grammar for PHP, which makes it hard to predict things like this, without simply trying them out. It's the way of the PHP world :)

Answer 2

PHP expects a variable or name reference for object definitions, it simply won't allow strings. You can use this hack which is based on '' empty string named variable the is instantly created and used based on the fact that ${false} evaluates to ${''} as follows to create an object from string:

$obj = new ${!${''} = 'App\MyClass'}();