Using namespaces: fallback to global function/constant
FAQ: things you need to know about namespaces
Namespaces
PHP Manual

## Name resolution rules

For the purposes of these resolution rules, here are some important definitions:

Namespace name definitions
Unqualified name

  This is an identifier without a namespace separator, such as Foo

Qualified name

  This is an identifier with a namespace separator, such as Foo\Bar

Fully qualified name

  This is an identifier with a namespace separator that begins with a namespace separator, such as \Foo\Bar. namespace\Foo is also a fully qualified name.

Names are resolved following these resolution rules:

1. Calls to fully qualified functions, classes or constants are resolved at compile-time. For instance new \A\B resolves to class A\B.
2. All unqualified and qualified names (not fully qualified names) are translated during compilation according to current import rules. For example, if the namespace A\B\C is imported as C, a call to C\D\e() is translated to A\B\C\D\e().
3. Inside a namespace, all qualified names not translated according to import rules have the current namespace prepended. For example, if a call to C\D\e() is performed within namespace A\B, it is translated to A\B\C\D\e().
4. Unqualified class names are translated during compilation according to current import rules (full name substituted for short imported name). In example, if the namespace A\B\C is imported as C, new C() is translated to new A\B\C().
5. Inside namespace (say A\B), calls to unqualified functions are resolved at run-time. Here is how a call to function foo() is resolved:
   1. It looks for a function from the current namespace: A\B\foo().
   2. It tries to find and call the global function foo().
6. Inside namespace (say A\B), calls to unqualified or qualified class names (not fully qualified class names) are resolved at run-time. Here is how a call to new C() or new D\E() is resolved. For new C():
   1. It looks for a class from the current namespace: A\B\C.
   2. It attempts to autoload A\B\C.
   For new D\E():
   1. It looks for a class by prepending the current namespace: A\B\D\E.
   2. It attempts to autoload A\B\D\E.
   To reference any global class in the global namespace, its fully qualified name new \C() must be used.

Example #1 Name resolutions illustrated

`<?php
namespace A;
use B\D, C\E as F;
// function calls
foo(); // first tries to call "foo" defined in namespace "A"
// then calls global function "foo"
\foo(); // calls function "foo" defined in global scope
my\foo(); // calls function "foo" defined in namespace "A\my"
F(); // first tries to call "F" defined in namespace "A"
// then calls global function "F"
// class references
new B(); // creates object of class "B" defined in namespace "A"
// if not found, it tries to autoload class "A\B"
new D(); // using import rules, creates object of class "D" defined in namespace "B"
// if not found, it tries to autoload class "B\D"
new F(); // using import rules, creates object of class "E" defined in namespace "C"
// if not found, it tries to autoload class "C\E"
new \B(); // creates object of class "B" defined in global scope
// if not found, it tries to autoload class "B"
new \D(); // creates object of class "D" defined in global scope
// if not found, it tries to autoload class "D"
new \F(); // creates object of class "F" defined in global scope
// if not found, it tries to autoload class "F"
// static methods/namespace functions from another namespace
B\foo(); // calls function "foo" from namespace "A\B"
B::foo(); // calls method "foo" of class "B" defined in namespace "A"
// if class "A\B" not found, it tries to autoload class "A\B"
D::foo(); // using import rules, calls method "foo" of class "D" defined in namespace "B"
// if class "B\D" not found, it tries to autoload class "B\D"
\B\foo(); // calls function "foo" from namespace "B"
\B::foo(); // calls method "foo" of class "B" from global scope
// if class "B" not found, it tries to autoload class "B"
// static methods/namespace functions of current namespace
A\B::foo(); // calls method "foo" of class "B" from namespace "A\A"
// if class "A\A\B" not found, it tries to autoload class "A\A\B"
\A\B::foo(); // calls method "foo" of class "B" from namespace "A\B"
// if class "A\B" not found, it tries to autoload class "A\B"
?>`

Using namespaces: fallback to global function/constant
FAQ: things you need to know about namespaces
Namespaces
PHP Manual