Namespaces

Question 1

What two problems are namespaces designed to solve in PHP?

Answer 1

1. Name collisions between code you create, and internal PHP classes/functions/constants or third-party classes/functions/constants.
2. Ability to alias (or shorten) Extra_Long_Names designed to alleviate the first problem, improving readability of source code.

Question 2

What are namespaces in PHP?

Answer 2

A way to group related classes, interfaces, functions and constants.

Question 3

What namespace names are reserved for internal language?

Answer 3

Namespace names PHP and php, and compound names starting with these names (like PHP\Classes).

Question 4

Which types of code are affected by namespaces?

Answer 4

Classes (including abstracts and traits), interfaces, functions and constants.

Question 5

How are namespaces declared?

Answer 5

With the “namespace” keyword. A file containing a namespace must declare the namespace at the top of the file before any other code - with one exception: the “declare” keyword, for defining encoding of a source file.

Question 6

Can non-php code precede a namespace declaration?

Answer 6

No. Not even extra whitespace.

Question 7

Can the same namespace be defined in multiple files?

Answer 7

Yes.

Question 8

Can namespaces be defined with sublevels, to specify a hierarchy of namespace names?

Answer 8

Yes, like this:

namespace MyProject\Sub\Level;

Question 9

Can multiple namespaces be declared in the same file?

Answer 9

Yes. There are two allowed syntaxes. With one you just declare multiple namespaces in a file. With the other, recommended method, you declare it like a function, with the namespace name followed by a braced section of code.

Question 10

How can global non-namespaced code be combined with namespaced code in a single file?

Answer 10

By using bracketed syntax. Global code should be enclosed in a namespace statement with no namespace name, just the “namespace” keyword.

Question 11

Regarding namespaces, what are the three ways that a class can be referred to in PHP?

Answer 11

1. Unqualified name: $a = new foo() or foo::staticmethod(). If the current namespace is currentnamespace, this resolves to currentnamespace\foo.
2. Qualified name: $a = new subnamespace\foo() or subnamespace\foo::staticmethod(). If the current namespace is currentnamespace, this resolves to currentnamespace\subnamespace\foo.
3. Fully qualified name: $a = new \currentnamespace\foo() or \currentnamespace\foo::staticmethod(). This always resolves to the literal name specified in the code.

Question 12

How can you access global classes, functions, or constants from within a namespace?

Answer 12

Use a fully qualified name, like \strlen() or \INI_ALL.

Question 13

What is the value of __NAMESPACE__?

Answer 13

A string that contains the current namespace name. In global, un-namespaced code, it contains an empty string.

Question 14

What are the three kinds of aliasing or importing that PHP supports with namespaces?

Answer 14

Aliasing a class name, aliasing an interface name, and aliasing a namespace name.

Question 15

How is aliasing accomplished?

Answer 15

With the “use” operator.

Question 16

For fully qualified namespace names, is the leading backslash necessary?

Answer 16

No, and it’s not recommended. Import names must be fully qualified and are not processed relative to the current namespace.

Question 17

Can multiple use statements be placed on the same line?

Answer 17

Yes, separated by commas.

Question 18

Are fully qualified names affected by importing?

Answer 18

No, fully qualified names are absolute, and unaffected by imports.

Question 19

Where must the use keyword be declared?

Answer 19

In the outermost scope of a file (the global scope), or inside namespace declarations. This is because importing is done at compile time, not runtime, so it cannot be block scoped.

Question 20

Without any namespace definition, in what scope are class and function definitions?

Answer 20

Global.

Question 21

What does prefixing a name with \ do?

Answer 21

Requires that it be called from the global space, even in the context of a namespace.

Question 22

Do class names resolve globally, or to the current namespace name?

Answer 22

To the current namespace name. So they will draw a fatal error if not found. However, functions and constants will fall back to global if a namespaced equivalent does not exist.

Question 23

What is an unqualified namespace name?

Answer 23

An identifier without a namespace separator: Foo.

Question 24

What is a qualified namespace name?

Answer 24

An identifier with a namespace separator: Foo\Bar.

Question 25

What is a fully qualified name?

Answer 25

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

Question 26

What are the name resolution rules?

Answer 26

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.

Question 27

How does a namespace name like my\name resolve?

Answer 27

Names that contain a backslash but do not begin with a backslash like my\name can be resolved in 2 different ways.

If there is an import statement that aliases another name to my, then the import alias is applied to the my in my\name.

Otherwise, the current namespace name is prepended to my\name.

Question 28

How does an unqualified class name like “name” resolve?

Answer 28

Class names that do not contain a backslash like name can be resolved in 2 different ways.

If there is an import statement that aliases another name to name, then the import alias is applied.

Otherwise, the current namespace name is prepended to name.

Question 29

How does an unqualified function name or unqualified constant name like “name” resolve?

Answer 29

Function or constant names that do not contain a backslash like name can be resolved in 2 different ways.

First, the current namespace name is prepended to name.

Finally, if the constant or function name does not exist in the current namespace, a global constant or function name is used if it exists.

Question 30

Can functions or constants be imported via the “use” statement?

Answer 30

No. The only elements that are affected by “use” statements are namespaces and class names.

Question 31

Can namespaces be used to override special constants: NULL, TRUE, FALSE, etc.?

Answer 31

No.