PHP average calculator not displaying results properly - php

Question

I'm creating a php calculator that needs to use the following classes, then print off the users name and the average score they achieved. This is the code I have so far, but it's not displaying correctly, it's saying there are missing arguments and undefined variables but i'm not sure where i've gone wrong!
<?php
class person {
public $name;
}
class student extends person {
function student ($name, $grade1, $grade2) {
if(is_numeric($grade1) && is_numeric($grade2)){
$grades = array($grade1, $grade2);
}
elseif (is_numeric($grade1)) {
$grade2 = 0;
}
else {
$grade1 = 0;
}
}
function average ($grades) {
$length = count($grades);
$total = 0;
for ($i = 0; $i < $length; $i++) {
$total = $total + $grades[i];
}
$average = $total / $length;
return $average;
}
}
$person1 = new student ($_POST['firstName'], $_POST['firstGrade1'], $_POST['firstGrade2']);
$person2 = new student ($_POST['secondName'], $_POST['secondGrade1'], $_POST['secondGrade2']);
$person3 = new student ($_POST['thirdName'], $_POST['thirdGrade1'], $_POST['thirdGrade2']);
echo "<br/> $person1->name" . "achieved an average of" . "$person1->average();";
echo "<br/> $person2->name" . "achieved an average of" . "$person2->average();";
echo "<br/> $person3->name" . "achieved an average of" . "$person3->average();";
?>
ERROR MESSAGES:
Warning: Missing argument 1 for student::average(), called in C:\Program Files (x86)\EasyPHP-DevServer-14.1VC11\data\localweb\my portable files\Exercise 4\average.php on line 40 and defined in C:\Program Files (x86)\EasyPHP-DevServer-14.1VC11\data\localweb\my portable files\Exercise 4\average.php on line 22
Notice: Undefined variable: grades in C:\Program Files (x86)\EasyPHP-DevServer-14.1VC11\data\localweb\my portable files\Exercise 4\average.php on line 23
Warning: Division by zero in C:\Program Files (x86)\EasyPHP-DevServer-14.1VC11\data\localweb\my portable files\Exercise 4\average.php on line 30

Answer 1

You don't appear to be returning that $grades variable. It's probably not defined because you aren't returning anything.
Your method:
function student ($name, $grade1, $grade2) {
if(is_numeric($grade1) && is_numeric($grade2)){
$grades = array($grade1, $grade2);
}
elseif (is_numeric($grade1)) {
$grade2 = 0;
$grades = array($grade1, $grade2);
}
else {
$grade1 = 0;
$grades = array($grade1, $grade2);
}
return $grades
}
Will need to look more like that. You'll also want to actually add grade1 and grad2 to your returned array in your alternate conditions.

Answer 2

The following is the classes enhanced. You will notice that they have constructors and functions to request variables. Private variables are usually the preferred method as they are protected and therefore cannot be modified outside the class.
class person {
private $name = "";
public function __construct ($nameV) {
$this->name = $nameV;
}
public function getName() {
return $this->name;
}
}
class student extends person {
private $grades;
public function __construct ($name, $grade1, $grade2) {
parent::__construct($name);
if ( ! is_numeric($grade1)) { $grade1 = 0; }
if ( ! is_numeric($grade2)) { $grade2 = 0; }
$this->grades = array($grade1, $grade2);
}
public function average () {
$length = count($this->grades);
$total = 0;
for ($i = 0; $i < $length; $i++) {
$total = $total + $this->grades[$i];
}
$average = $total / $length;
return $average;
}
}
The export code is then simply:
echo "<br/>" . $person1->getName() . " achieved an average of " . $person1->average();
echo "<br/>" . $person2->getName() . " achieved an average of " . $person2->average();
echo "<br/>" . $person3->getName() . " achieved an average of " . $person3->average();
Due to not having the form, I tested with the following data:
$person1 = new student ("XYZ", 1, 2);
$person2 = new student ("XYZ2", 100, 20);
$person3 = new student ("XYZ3", 95, 94);
Which exported:
XYZ achieved an average of 1.5
XYZ2 achieved an average of 60
XYZ3 achieved an average of 94.5