請看下面的代碼：
復制代碼 代碼如下:
<?php
class A {
public function x() {
echo "A::x() was called.\n";
}
public function y() {
self::x();
echo "A::y() was called.\n";
}
public function z() {
$this->x();
echo "A::z() was called.\n";
}
}
class B extends A {
public function x() {
echo "B::x() was called.\n";
}
}
$b = new B();
$b->y();
echo "--\n";
$b->z();
?>
該例中，A::y()調用了A::x()，而B::x()覆蓋了A::x()，那麼當調用B::y()時，B::y()應該調用A::x()還是 B::x()呢？在C++中，如果A::x()未被定義為虛函數，那麼B::y()（也就是A::y()）將調用A::x()，而如果A::x()使用 virtual關鍵字定義成虛函數，那麼B::y()將調用B::x()。然而，在PHP5中，虛函數的功能是由 self 和 $this 關鍵字實現的。如果父類中A::y()中使用 self::x() 的方式調用了 A::x()，那麼在子類中不論A::x()是否被覆蓋，A::y()調用的都是A::x()；而如果父類中A::y()使用 $this->x() 的方式調用了 A::x()，那麼如果在子類中A::x()被B::x()覆蓋，A::y()將會調用B::x()。
上例的運行結果如下：
A::x() was called. A::y() was called. --
B::x() was called. A::z() was called.
virtual-function.php
復制代碼 代碼如下:
<?php
class ParentClass {
static public function say( $str ) {
static::do_print( $str );
}
static public function do_print( $str ) {
echo "<p>Parent says $str</p>";
}
}
class ChildClass extends ParentClass {
static public function do_print( $str ) {
echo "<p>Child says $str</p>";
}
}
class AnotherChildClass extends ParentClass {
static public function do_print( $str ) {
echo "<p>AnotherChild says $str</p>";
}
}
echo phpversion();
$a=new ChildClass();
$a->say( 'Hello' );
$b=new AnotherChildClass();
$b->say( 'Hello' );
