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<h2 class="title">Namespaces overview</h2>
<p class="verinfo">(PHP 5 >= 5.3.0)</p>
<p class="simpara">
What are namespaces? In the broadest definition namespaces are a way of encapsulating
items. This can be seen as an abstract concept in many places. For example, in any
operating system directories serve to group related files, and act as a namespace for
the files within them. As a concrete example, the file <em>foo.txt</em> can
exist in both directory <em>/home/greg</em> and in <em>/home/other</em>,
but two copies of <em>foo.txt</em> cannot co-exist in the same directory. In
addition, to access the <em>foo.txt</em> file outside of the
<em>/home/greg</em> directory, we must prepend the directory name to the file
name using the directory separator to get <em>/home/greg/foo.txt</em>. This
same principle extends to namespaces in the programming world.
</p>
<p class="simpara">
In the PHP world, namespaces are designed to solve two problems that authors
of libraries and applications encounter when creating re-usable code elements
such as classes or functions:
</p>
<p class="para">
<ol type="1">
<li class="listitem">
<span class="simpara">
Name collisions between code you create, and
internal PHP classes/functions/constants or third-party classes/functions/constants.
</span>
</li>
<li class="listitem">
<span class="simpara">
Ability to alias (or shorten) Extra_Long_Names designed to alleviate the first problem,
improving readability of source code.
</span>
</li>
</ol>
</p>
<p class="simpara">
PHP Namespaces provide a way in which to group related classes, interfaces,
functions and constants. Here is an example of namespace syntax in PHP:
</p>
<div class="example" id="example-233">
<p><strong>Example #1 Namespace syntax example</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">namespace </span><span style="color: #0000BB">my</span><span style="color: #007700">\</span><span style="color: #0000BB">name</span><span style="color: #007700">; </span><span style="color: #FF8000">// see "Defining Namespaces" section<br /><br /></span><span style="color: #007700">class </span><span style="color: #0000BB">MyClass </span><span style="color: #007700">{}<br />function </span><span style="color: #0000BB">myfunction</span><span style="color: #007700">() {}<br />const </span><span style="color: #0000BB">MYCONST </span><span style="color: #007700">= </span><span style="color: #0000BB">1</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">$a </span><span style="color: #007700">= new </span><span style="color: #0000BB">MyClass</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$c </span><span style="color: #007700">= new \</span><span style="color: #0000BB">my</span><span style="color: #007700">\</span><span style="color: #0000BB">name</span><span style="color: #007700">\</span><span style="color: #0000BB">MyClass</span><span style="color: #007700">; </span><span style="color: #FF8000">// see "Global Space" section<br /><br /></span><span style="color: #0000BB">$a </span><span style="color: #007700">= </span><span style="color: #0000BB">strlen</span><span style="color: #007700">(</span><span style="color: #DD0000">'hi'</span><span style="color: #007700">); </span><span style="color: #FF8000">// see "Using namespaces: fallback to global<br /> // function/constant" section<br /><br /></span><span style="color: #0000BB">$d </span><span style="color: #007700">= namespace\</span><span style="color: #0000BB">MYCONST</span><span style="color: #007700">; </span><span style="color: #FF8000">// see "namespace operator and __NAMESPACE__<br /> // constant" section<br /></span><span style="color: #0000BB">$d </span><span style="color: #007700">= </span><span style="color: #0000BB">__NAMESPACE__ </span><span style="color: #007700">. </span><span style="color: #DD0000">'\MYCONST'</span><span style="color: #007700">;<br />echo </span><span style="color: #0000BB">constant</span><span style="color: #007700">(</span><span style="color: #0000BB">$d</span><span style="color: #007700">); </span><span style="color: #FF8000">// see "Namespaces and dynamic language features" section<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
Namespace names <em>PHP</em> and <em>php</em>, and compound names starting
with these names (like <em>PHP\Classes</em>) are reserved for internal language use
and should not be used in the userspace code.
</p>
</p></blockquote>
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