What is the difference between `while (true)` vs `while (\true)`?

Question

What is the difference between while (true) vs while (\true) and most Importantly can anyone clarify why boolean are affected by namespace in PHP ?

Answer 1

In PHP true, false and null are constants, which are protected from being overwritten in namespaces. As such the following is invalid code:

namespace Foo; const true = false; // Cannot redeclare constant 'true' 

It is still possible to overwrite such a constant using ''define()'':

namespace Foo; define('Foo\true', false); var_dump(true); // bool(false) 

However this isn't supported functionality (it might be called a bug) and PHP is free to assume that true cannot be overwritten. For example usage of true in a constexpr context will not be affected by the above definition:

// Note: This uses eval() to make sure the define() runs before the constexpr //       constant resolution happens  namespace Foo; define('Foo\true', false); var_dump(eval('namespace Foo; static $t = true; return $t;')); // bool(true) 

The reason why the substitution happens in the constexpr case, but not in the "normal" case is a bug in the implementation. For constexprs the substitution happens before name resolution, whereas for normal code it happens after name resolution.

The correct implementation would be to always substitute after name resolution, but specifically account for true, false and null during the substitution. I plan to fix this for PHP 7.