Undefined variable notices in PHP class instance call

Undefined variable notices in PHP class instance call

Using PHP, when I call an instance of a class I am getting an undefined
variable notice back. I program in other languages, but am less familiar
with PHP, can you please look at my code and inform me as to my mistake in
defining my class variables? Thanks.
<?php
// create class Bike
class Bike
{
// create variables for class Bike of price, max_speed, and miles
var $price;
var $max_speed;
var $miles;
// create constructor for class Bike with user set price and
max_speed
function __construct($price, $max_speed)
{
$this->price = $price;
$this->max_speed = $max_speed;
}
// METHODS for class Bike:
// displayInfo() - have this method display the bike's price,
maximum speed, and the total miles driven
function displayInfo()
{
echo "Price: $" . $price . "<p>Maximum Speed: " . $max_speed
. "</p>" . "<p>Total Miles Driven: " . $miles . "</p>";
}
// drive() - have it display "Driving" on the screen and increase
the total miles driven by 10
function drive()
{
$miles = $miles + 10;
echo "<p>Driving!</p>" . "<p>Total Miles Driven: " . $miles .
"</p>";
}
// reverse() - have it display "Reversing" on the screen and
decrease the total miles driven by 5...
function reverse()
{
// What would you do to prevent the instance from having
negative miles?
if($miles >= 5)
{
$miles = $miles - 5;
echo "<p>Reversing</p><p>Total Miles Driven: " . $miles .
"</p>";
}
else
{
echo "<p>Total Miles Driven is less than 5 miles, unable
to reverse!</p>";
}
}
}
?>
<?php
// Create 3 bike object class instances
$Schwinn = new Bike(50, '10mph');
$Specialized = new Bike(500, '25mph');
$Cannondale = new Bike(1000, '50mph');
// Have the first instance drive three times, reverse one and have it
displayInfo().
var_dump($Schwinn); // shows instance is created with proper variable
assignments
$Schwinn -> drive();
// $Schwinn -> drive();
// $Schwinn -> drive();
$Schwinn -> reverse();
$Schwinn -> displayInfo();
?>