PHP OOP: Accessing Variables

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

Note -- don't use global to access the $db resource. Pass it as an argument to the method, or (as likely your static methods will become instance, based on the answers here) pass it to the constructor of the object. — Dan Lugg
– Dan Lugg, Commented Dec 31, 2011 at 1:02

Saad Imran. · Accepted Answer · 2011-12-31 01:00:38Z

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.

answered Dec 31, 2011 at 1:00

Saad Imran.

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Comments

KingCrunch · Accepted Answer · 2011-12-31 01:01:46Z

1

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 */ }

answered Dec 31, 2011 at 1:01

KingCrunch

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Telmo Marques · Accepted Answer · 2011-12-31 01:00:31Z

0

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.

answered Dec 31, 2011 at 1:00

Telmo Marques

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Comments

Prasad Rajapaksha · Accepted Answer · 2011-12-31 01:10:38Z

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 !

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