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</div><hr /><div id="language.oop5.abstract" class="sect1">
<h2 class="title">Class Abstraction</h2>
<p class="para">
PHP 5 introduces abstract classes and methods. Classes defined as
abstract may not be instantiated, and any class that
contains at least one abstract method must also be abstract. Methods
defined as abstract simply declare the method's signature - they cannot
define the implementation.
</p>
<p class="para">
When inheriting from an abstract class, all methods marked abstract in
the parent's class declaration must be defined by the child; additionally,
these methods must be defined with the same (or a less restricted)
<a href="language.oop5.visibility.html" class="link">visibility</a>. For example,
if the abstract method is defined as protected, the function implementation
must be defined as either protected or public, but not private. Furthermore
the signatures of the methods must match, i.e. the type hints and the number
of required arguments must be the same. For example, if the child class
defines an optional argument, where the abstract method's signature does
not, there is no conflict in the signature. This also applies to constructors
as of PHP 5.4. Before 5.4 constructor signatures could differ.
</p>
<div class="example" id="example-196">
<p><strong>Example #1 Abstract class 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">abstract class </span><span style="color: #0000BB">AbstractClass<br /></span><span style="color: #007700">{<br /> </span><span style="color: #FF8000">// Force Extending class to define this method<br /> </span><span style="color: #007700">abstract protected function </span><span style="color: #0000BB">getValue</span><span style="color: #007700">();<br /> abstract protected function </span><span style="color: #0000BB">prefixValue</span><span style="color: #007700">(</span><span style="color: #0000BB">$prefix</span><span style="color: #007700">);<br /><br /> </span><span style="color: #FF8000">// Common method<br /> </span><span style="color: #007700">public function </span><span style="color: #0000BB">printOut</span><span style="color: #007700">() {<br /> print </span><span style="color: #0000BB">$this</span><span style="color: #007700">-></span><span style="color: #0000BB">getValue</span><span style="color: #007700">() . </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /> }<br />}<br /><br />class </span><span style="color: #0000BB">ConcreteClass1 </span><span style="color: #007700">extends </span><span style="color: #0000BB">AbstractClass<br /></span><span style="color: #007700">{<br /> protected function </span><span style="color: #0000BB">getValue</span><span style="color: #007700">() {<br /> return </span><span style="color: #DD0000">"ConcreteClass1"</span><span style="color: #007700">;<br /> }<br /><br /> public function </span><span style="color: #0000BB">prefixValue</span><span style="color: #007700">(</span><span style="color: #0000BB">$prefix</span><span style="color: #007700">) {<br /> return </span><span style="color: #DD0000">"</span><span style="color: #007700">{</span><span style="color: #0000BB">$prefix</span><span style="color: #007700">}</span><span style="color: #DD0000">ConcreteClass1"</span><span style="color: #007700">;<br /> }<br />}<br /><br />class </span><span style="color: #0000BB">ConcreteClass2 </span><span style="color: #007700">extends </span><span style="color: #0000BB">AbstractClass<br /></span><span style="color: #007700">{<br /> public function </span><span style="color: #0000BB">getValue</span><span style="color: #007700">() {<br /> return </span><span style="color: #DD0000">"ConcreteClass2"</span><span style="color: #007700">;<br /> }<br /><br /> public function </span><span style="color: #0000BB">prefixValue</span><span style="color: #007700">(</span><span style="color: #0000BB">$prefix</span><span style="color: #007700">) {<br /> return </span><span style="color: #DD0000">"</span><span style="color: #007700">{</span><span style="color: #0000BB">$prefix</span><span style="color: #007700">}</span><span style="color: #DD0000">ConcreteClass2"</span><span style="color: #007700">;<br /> }<br />}<br /><br /></span><span style="color: #0000BB">$class1 </span><span style="color: #007700">= new </span><span style="color: #0000BB">ConcreteClass1</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$class1</span><span style="color: #007700">-></span><span style="color: #0000BB">printOut</span><span style="color: #007700">();<br />echo </span><span style="color: #0000BB">$class1</span><span style="color: #007700">-></span><span style="color: #0000BB">prefixValue</span><span style="color: #007700">(</span><span style="color: #DD0000">'FOO_'</span><span style="color: #007700">) .</span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">$class2 </span><span style="color: #007700">= new </span><span style="color: #0000BB">ConcreteClass2</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$class2</span><span style="color: #007700">-></span><span style="color: #0000BB">printOut</span><span style="color: #007700">();<br />echo </span><span style="color: #0000BB">$class2</span><span style="color: #007700">-></span><span style="color: #0000BB">prefixValue</span><span style="color: #007700">(</span><span style="color: #DD0000">'FOO_'</span><span style="color: #007700">) .</span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents"><p>The above example will output:</p></div>
<div class="example-contents screen">
<div class="cdata"><pre>
ConcreteClass1
FOO_ConcreteClass1
ConcreteClass2
FOO_ConcreteClass2
</pre></div>
</div>
</div>
<div class="example" id="example-197">
<p><strong>Example #2 Abstract class 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">abstract class </span><span style="color: #0000BB">AbstractClass<br /></span><span style="color: #007700">{<br /> </span><span style="color: #FF8000">// Our abstract method only needs to define the required arguments<br /> </span><span style="color: #007700">abstract protected function </span><span style="color: #0000BB">prefixName</span><span style="color: #007700">(</span><span style="color: #0000BB">$name</span><span style="color: #007700">);<br /><br />}<br /><br />class </span><span style="color: #0000BB">ConcreteClass </span><span style="color: #007700">extends </span><span style="color: #0000BB">AbstractClass<br /></span><span style="color: #007700">{<br /><br /> </span><span style="color: #FF8000">// Our child class may define optional arguments not in the parent's signature<br /> </span><span style="color: #007700">public function </span><span style="color: #0000BB">prefixName</span><span style="color: #007700">(</span><span style="color: #0000BB">$name</span><span style="color: #007700">, </span><span style="color: #0000BB">$separator </span><span style="color: #007700">= </span><span style="color: #DD0000">"."</span><span style="color: #007700">) {<br /> if (</span><span style="color: #0000BB">$name </span><span style="color: #007700">== </span><span style="color: #DD0000">"Pacman"</span><span style="color: #007700">) {<br /> </span><span style="color: #0000BB">$prefix </span><span style="color: #007700">= </span><span style="color: #DD0000">"Mr"</span><span style="color: #007700">;<br /> } elseif (</span><span style="color: #0000BB">$name </span><span style="color: #007700">== </span><span style="color: #DD0000">"Pacwoman"</span><span style="color: #007700">) {<br /> </span><span style="color: #0000BB">$prefix </span><span style="color: #007700">= </span><span style="color: #DD0000">"Mrs"</span><span style="color: #007700">;<br /> } else {<br /> </span><span style="color: #0000BB">$prefix </span><span style="color: #007700">= </span><span style="color: #DD0000">""</span><span style="color: #007700">;<br /> }<br /> return </span><span style="color: #DD0000">"</span><span style="color: #007700">{</span><span style="color: #0000BB">$prefix</span><span style="color: #007700">}{</span><span style="color: #0000BB">$separator</span><span style="color: #007700">}</span><span style="color: #DD0000"> </span><span style="color: #007700">{</span><span style="color: #0000BB">$name</span><span style="color: #007700">}</span><span style="color: #DD0000">"</span><span style="color: #007700">;<br /> }<br />}<br /><br /></span><span style="color: #0000BB">$class </span><span style="color: #007700">= new </span><span style="color: #0000BB">ConcreteClass</span><span style="color: #007700">;<br />echo </span><span style="color: #0000BB">$class</span><span style="color: #007700">-></span><span style="color: #0000BB">prefixName</span><span style="color: #007700">(</span><span style="color: #DD0000">"Pacman"</span><span style="color: #007700">), </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br />echo </span><span style="color: #0000BB">$class</span><span style="color: #007700">-></span><span style="color: #0000BB">prefixName</span><span style="color: #007700">(</span><span style="color: #DD0000">"Pacwoman"</span><span style="color: #007700">), </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents"><p>The above example will output:</p></div>
<div class="example-contents screen">
<div class="cdata"><pre>
Mr. Pacman
Mrs. Pacwoman
</pre></div>
</div>
</div>
<p class="para">
Old code that has no user-defined classes or functions named
'abstract' should run without modifications.
</p>
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