New to OOP - Help with Small Piece of Code

[carsonk]

Hi! 🙂

I just started a new project and I decided I would try doing it with Object-Oriented PHP instead of Procedural. I learned the basics, and I started off writing a small piece of code to retrieve user information from a database. Though, I am receiving an error that I don't really understand, and I still don't believe the code is as efficient as it could be for what I want to do.

Here it is:

<?php

$mysqli = new mysqli('localhost', 'PRIVATE', 'PRIVATE', 'PRIVATE');

if (mysqli_connect_error()) {
    die('Connect Error (' . mysqli_connect_errno() . ') '
            . mysqli_connect_error());
}

class user {
	var $id;
	var $username;

//when using, type (referring to input) can either be 'id' or 'username'
//input is either the user id or username, depending on which you picked for the type
public function __construct($input, $type) {
	if ($type == 'username') {
		$sql = 'SELECT * FROM users WHERE username = '.$input;
		$res = $mysqli->query($sql);
		$row = $res->fetch_array();
		$this->id = $row["id"];
		$this->username = $input;
	} else if($type = 'id') {

	}
}

public function id() {
	return $this->id;
}

public function username() {
	return $this->username;
}
}

$user = new user('test', 'username');
echo $user->id();

?>

And I received the following error:

Fatal error: Call to a member function query() on a non-object in tng/common.php on line 19

Line 19 being:

$res = $mysqli->query($sql);

Any help is highly appreciated.

[carsonk]

Just thought of this; do I have to make "$mysqli" a global variable in the function?

[NogDog]

carsonk;10940249 wrote:

Just thought of this; do I have to make "$mysqli" a global variable in the function?

While that would likely work, it's not generally considered good technique to use global within functions/methods, as it has maintenance, testing, and re-usability issues. I would probably pass it as a parameter to the constructor.

class User
{
   private $db;
// . . . //
   public function __construct($input, $type, MySQLi $db) { // can use type-hinting
      $this->db = $db;
      // rest of constructor...
   }

   public function whatever()
   {
      $this->db->query("SELECT * FROM whatever");
   }
}

[halojoy]

$mysqli = new mysqli('localhost', 'PRIVATE', 'PRIVATE', 'PRIVATE'); 

So $mysqli is a global

Think you could make it global inside function.
Or you could send it as an argument:
$user = new user($mysqli, 'test', 'username');
and in Class make a variable for it:

class user {
    var $id;
    var $username; 
    var $db;
/////

 $this->db = $mysqli;
/////
            $sql = 'SELECT * FROM users WHERE username = '.$input;
            $res = $this->db->query($sql); 

[Kudose]

I agree with NogDog. Classes should not be dependent on global variables.

[nrg_alpha]

halojoy;10940257 wrote:

$mysqli = new mysqli('localhost', 'PRIVATE', 'PRIVATE', 'PRIVATE'); 

So $mysqli is a global

That object would be global only if it is created within global space (outside of any other classes). If a class is instantiated within another class, that new object is restricted to class scope, and as such, does not exist outside of it.

Example:

class Foo {
    private $_obj;
    public function __construct(){
	   $_obj = new Bar(7);
    }
}

class Bar {
    private $_age;
    public function __construct($t){
	   $this->_age = $t;
    }

public function get_age() {
   return $this->_age;
}
}

$whatEver = new Foo(); // object $whatEver is global, as it is instantiated from class Foo in global space
echo $whatEver->$obj->get_age(); // fail. $_obj is instantiated from within class Foo and is thus restricted to class scope (local)
echo $obj->get_age(); // This too would fail, as obj woud be considered an undefined variable

If you added say echo $obj->get_age(); within the Foo constructor, this would work (the value of $obj's property $age would be fetched).