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content/12-classes-inheritance.md

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+In PHP, a class can extend another class, inheriting the parent class'
+properties and methods. To make a class a child of another, use the `extends`
+keyword after the class name.
+```php
+<?php
+
+class Vehicle
+{
+    public function drive()
+    {
+        echo "driving...\n";
+    }
+}
+
+class Truck extends Vehicle {}
+```
+
+Using the `drive` method on the `Truck` class does not cause an error because `Truck` extends `Vehicle`.
+```php
+$truck = new Truck();
+$truck->drive();
+```
+
+Even though the child class inherits a parent class' properties and methods,
+the child can still override the parent.
+```php
+class Tractor extends Vehicle
+{
+    public function drive()
+    {
+        echo "driving slowly...\n";
+    }
+}
+```
+
+The drive function now outputs "driving slowly..." instead of "driving...".
+```php
+$tractor = new Tractor();
+$tractor->drive();
+```
+
+A class can use a parent's property or method from the `$this` variable.
+```php
+class Motorcycle extends Vehicle
+{
+    public function pushPedal()
+    {
+        $this->drive();
+    }
+}
+```
+
+The `pushPedal` method outputs "driving...".
+```php
+$cycle = new Motorcycle();
+$cycle->pushPedal();
+```
+
+If you override a parent's property or method, the `$this` variable will refer to the child's
+implementation of the property or method. To call the parent's property or method explicity,
+use the `parent` keyword.
+```php
+class Racecar extends Vehicle
+{
+    public function drive()
+    {
+        parent::drive();
+
+        echo "driving even faster...\n";
+    }
+}
+```
+
+The `drive` method on `Racecar` now outputs "driving..." and "driving even faster...".
+```php
+$racecar = new Racecar();
+$racecar->drive();
+```