What are NAMESPACES and how works?

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Namespacing does for functions and classes what scope does for variables. 
It allows you to use the same function or class name in different parts of the
same program without causing a name collision.

In simple terms, think of a namespace as a person's surname. If there are two people n
amed "John" you can use their surnames to tell them apart.
The Scenario

Suppose you write an application that uses a function named output().
Your output() function takes all of the HTML code on your page and sends it to the user.

Later on your application gets bigger and you want to add new features. 
You add a library that allows you to generate RSS feeds. 
This library also uses a function named output() to output the final feed.

When you call output(), how does PHP know whether to use your output() function or the 
RSS library's output() function? It doesn't. Unless you're using namespaces.
Example

How do we solve having two output() functions? Simple. We stick each output() 
function in its own namespace.

That would look something like this:

namespace MyProject;

function output() {
    # Output HTML page
    echo 'HTML!';
}

namespace RSSLibrary;

function output(){
    # Output RSS feed
    echo 'RSS!';
}

Later when we want to use the different functions, we'd use:

\MyProject\output();
\RSSLibrary\output();

Or we can declare that we're in one of the namespaces and then we can 
just call that namespace's output():

namespace MyProject;

output(); # Output HTML page
\RSSLibrary\output();

No Namespaces?

If we didn't have namespaces we'd have to (potentially) change a lot of code
any time we added a library, or come up with tedious prefixes to make our function
names unique. With namespaces, we can avoid the headache of naming collisions when
mixing third-party code with our own projects.

testMe(); // Will output testMe() from first class