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    <p>It is useful if you want to apply <a href="http://en.wikipedia.org/wiki/Object-oriented_programming" rel="noreferrer">OOP</a> practices such as <a href="http://en.wikipedia.org/wiki/Information_hiding" rel="noreferrer">Information Hiding</a>(<a href="http://en.wikipedia.org/wiki/Encapsulation_(object-oriented_programming)" rel="noreferrer">Encapsulation</a>).</p> <p>If you declare class members as <code>private</code>, they cannot be accessed from code outside of your class. You have to provide <em>methods</em> to access them. This separates the interface of your class from the actual implementation.<br> Other code that uses your class does not need to know the name of the class member or <em>how</em> you actually store information internally.</p> <p>Example:</p> <p>Consider a class <code>Books</code> the somehow gives me a list of books. I can define a <code>public</code> member that holds an array of books:</p> <pre><code>class Books { public $list; } </code></pre> <p>On the other side, if I define a method <code>getList()</code>, I can change the implementation later, without effecting the code that uses the class:</p> <pre><code>class Books { private $list; public function getList() { // get list from database, from XML, etc. // might use $list internally but does not have to } } </code></pre> <p>Obviously, you don't <em>need</em> modifiers like <code>private</code>, <code>protected</code> or <code>public</code> to implement this behavior, but it leads to better structured code and clearer interfaces. Imagine you have a class that has both a <code>public $list</code> and a method <code>getList()</code>. How do you know which one of them to use?<br> <strong>This applies not only for <em>getting</em> values, but especially for <em>setting</em> values.</strong></p> <p>There are no disadvantages if you use them, only advantages IMHO. The difference between them is the scope of the visibility. <code>public</code> members can be accessed from outside code, <code>protected</code> members can be accessed form inheriting classes and <code>private</code> members only from the class.</p> <p>Methods can also have these modifiers and follow a similar purpose. E.g. if a method is declared as <code>private</code>, it is clear that it is probably some kind of helper method, that is used internally only and is not supposed to be called form the outside.</p> <hr> <p>So in the end it boils down to two things:</p> <ul> <li><strong>Control</strong>: Which parts of your class can be accessed in which way</li> <li><strong>Self-documentation or understanding</strong>: Other people using your class can figure out more easily, which part of your class they are supposed to access and which not.</li> </ul>
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    1. COYou seem to mix up C++ with PHP... :) Don't forget the to declare `getList` as an actual function (i.e.: `public function getList`).
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      1. USnetcoder
    2. CO@netcoder: Ah thanks :) If you switch between several programming languages it is easy to loose track sometimes ;)
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    3. COWow, excellent answer, man! +1!
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