PHP OOP: Accessing Variables - php

Question

I have the following "Student" Class:
class Student {
public $user_id;
public $name;
public function __construct($user_id) {
$info = $this->studentInfo($user_id);
$this->name = $info['name'];
$this->is_instructor = $info['is_instructor'];
$this->user_id = $info['id'];
}
public static function studentInfo($id) {
global $db;
$u = mysql_fetch_array(mysql_query("SELECT * FROM $db[students] WHERE id='$id'"));
if($u) {
return $u;
}
}
public static function getCoursesByInstructor() {
global $db;
return mysql_query("SELECT courses.*, course_types.name FROM $db[courses] as courses
JOIN $db[course_types] as course_types ON courses.course_type_id=course_types.id
WHERE instructor_id='$this->user_id'");
}
}
I'm trying to do:
$u = new Student(1);
$courses = $u->getCoursesByInstructor();
But am getting the following error:
Fatal error: Using $this when not in object context in /Applications/MAMP/htdocs/flight1/phpincludes/classes/students.class.php on line 54

Answer 1

You're getting that error because you're function is a static function, and therefore you cannot use the $this pointer within it because it supposed to point to an object. So just remove the static keyword from your function definitions.

Answer 2

You are using static methods non-statically. Static methods are only bound to its class, but not to an object, thus $this is not available. This especially means, that $this->userid, that you use in getCoursesByInstructor(), is not valid. I recommend to make the method non-static.
public function getCoursesByInstructor() { /* your code here */ }

Answer 3

Remove the static keyword from the functions you declared inside your Student class.
The static keyword is, simply put, used to indicate a function that is accessible without the need of creating an instance of that class. On the other hand, $this is used to refer to class instance variables. That's why those two don't go well togheter, you're trying to access an instance variable (by using $this) in a static context.

Answer 4

The issue with your code is you are requesting $this inside your static function. See this. You have $this->user_id inside the query.
return mysql_query("SELECT courses.*, course_types.name FROM $db[courses] as courses
JOIN $db[course_types] as course_types ON courses.course_type_id=course_types.id
WHERE instructor_id='$this->user_id'");
To fix this issue you have to modify this function. I can suggest you following way.
public static function getCoursesByInstructor($userID) {
global $db;
return mysql_query("SELECT courses.*, course_types.name FROM $db[courses] as courses
JOIN $db[course_types] as course_types ON courses.course_type_id=course_types.id
WHERE instructor_id='$userID'");
}
And you have to change your other functions with the same theory.
Cheers !