Did we talk about why we declare properties in a class with the visibility private? Answer; To protect the integrity of the properties. We, the authors of the class, know better how to do it. We don't want any user (program) to interfere in that, thus:

<?php
class Runner {
	private $name;	// name
	private $club;		// affiliation
	private $distance;	// race distance
	private $stride;	// length of each step in cms
	private $cadence;	// number of steps per minute
	
	public function __construct($name, $club, $distance, $stride, $cadence) {
		$this->name = $name;
		$this->club = $club;
		$this->distance = $distance;
		$this->stride = $stride;
		$this->cadence = $cadence;
	}
	
	public function getStride() {
		return $this->stride;
	}
	
	public function setStride($stride) {
		$this->stride = $stride;
	}

	public function getRunner() {
		return $this->name . ", " . $this->club;
	}
	
	public function runSpeed() {
		return $this->stride * $this->cadence / 100 * 60 / 1000;	// km/h
	}
}
?>

Exercise: Test the above class!

The normal procedure for a user is to ascertain that arguments sent into methods (or functions) are of the right type. In a dynamically typed language, this, of course, is a run-time issue, not a compile-time problem. PHP offers you a series of static functions to do just that:

<?php
bool is_bool(mixed $var)	// returns boolean, param mixed type
				// usage
	if (is_boo($var)) {
		echo "True";
	} else {
		echo "False";
	}
	
is_integer($var);	// returns true if $var is integer
is_double($var);	// returns true if $var is double
is_string($var);		// returns true if $var is character data 
is_object($var);	// returns true if $var is an object ref
is_array($var);		// returns true if $var is an array
is_resource($var);	// returns true if $var is a resource (file or db)
is_null($var);		// returns true if $var has no value
	

Now take the Runner from above. let us imagine that sometimes we need to gather runners for seminars and lectures, not necvessarily for races. The class Runner then seems quite ill suited. We need essentials, but not all.

Let's look at a class diagram ... draw on blackboard

Then we shall code the corresponding (new) classes

<?php
class Runner {
	private $name;	// name
	private $club;		// affiliation
	
	public function __construct($name, $club) {
		$this->name = $name;
		$this->club = $club;
	}
	
	public function getRunner() {
		return $this->name . ", " . $this->club;
	}
}
?>

The runner reduced to the bare person-related stuff, but now look. The RaceRunner IS_A Runner

<?php
class RaceRunner extends Runner {
	private $distance;	// race distance
	private $stride;	// length of each step in cms
	private $cadence;	// number of steps per minute
	
	public function __construct($name, $club, $distance, $stride, $cadence) {
		parent::__construct($name, $club);
		$this->distance = $distance;
		$this->stride = $stride;
		$this->cadence = $cadence;
	}
	
	public function getStride() {
		return $this->stride;
	}
	
	public function setStride($stride) {
		$this->stride = $stride;
	}
	
	public function runSpeed() {
		return $this->stride * $this->cadence / 100 * 60 / 1000;	// km/h
	}
}
?>

Usage:

<?php
...
include "Runner.inc.php";
include "RaceRunner.inc.php";
...
...
$seminarParticipant = new Runner("Niels", "AGF");	// runner only, not connected to race
printf("<br />%s\n", $seminarParticipant->getRunner());
...
...
$racer = new RaceRunner("Niels", "AGF", 12, 120, 180);
printf("<td>%s"</td><td>%s"</td>\n", $racer->getRunner(), $racer->runSpeed();
?>

Exercise: Complete the usage example, to demonstrate that it works.