PHP: "Call to undefined method" ... but method is defined?

Question

I have a class Database.php, which is an abstract Singleton class:

<?php

abstract class Database
{
    protected static $_instance;
    ...

    public static function instance()
    {
        $class = get_called_class();

        if (!self::$_instance) {
            self::$_instance = new $class();
        }

        return self::$_instance;
    }
}

?>

Other databases extend this class and implement the abstract functions, so that I can change the database I use, while making sure my application still works as long as it uses the functions in this abstraction layer (I have left out the function definitions from the above code).

I have a class PDO, in PDO.php. It extends database and also includes the instance() function identical to the one above, but it does not have it's own $_instance variable.

class PDO extends Database
{
    //...

    public static function instance()
    {
        $class = get_called_class();

        if (!self::$_instance) {
                self::$_instance = new $class();
        }

        return self::$_instance;
    }
}

Originally I thought I wouldn't have to include the instance() function in the PDO class, since it inherits from Database. But I included it because I was getting this error:

Fatal error: Call to undefined method PDO::instance()

The problem is that I'm still getting this error, even with the code above. I don't know if this is related, but there is another strange error I was getting. The type of database I want to use, in this case PDO, is stored in a variable accessible through App::setting('DB'). The Model base class in my MVC framework loads the appropriate database class and stores it in $this->_db. Here is the code for the Model:

<?php

require_once 'Databases/Database.php';

class Model
{
    protected $_db;

    public function __construct()
    {
        $database = App::setting('DB'); // 'PDO' 
        require_once 'Databases/' . $database . '.php';
        $this->_db = $database::instance(); // this is what triggers the error
    }
}

?>

Now this code was giving me an error that I could not redeclare the PDO class. I searched online, and found that the problem is usually related to using include() instead of require_once(). I checked everywhere and my Autoloader, didn't see any include(). Even with require_once() in the model above, it was still giving me the error (this is the only place I was requiring the PDO class...).

To fix that error, I am using a band-aid solution where I replace the require in the Model constructor with:

if (!class_exists($database)) {
    require_once 'Databases/' . $database . '.php';
}

Can anyone explain what is going on here?

By the way Logan, just in-case you didn't know, you can extend built-in classes too. It's not uncommon to see class Database extends PDO. Although you should stay away from singletons and use dependency injection where possible. — lll
– lll, Commented Jul 30, 2012 at 6:44
I don't want to extend PDO. I am using a Singleton because I only want one Database object for any model, so that if I were to load two models, they wouldn't connect to the database twice. Is there a better way to achieve this? I don't see how dependency injection would work but I'm no expert. A lot of PHP frameworks out there use Singletons, so that is what I used. — Logan Serman
– Logan Serman, Commented Jul 30, 2012 at 8:48

enricog · Accepted Answer · 2012-07-30 06:20:18Z

7

The class name PDO is a standard class of PHP, so with your solution to include your PDO class you are actually never including yours.

answered Jul 30, 2012 at 6:20

enricog

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3 Comments

lll Over a year ago

That would give a different error. Cannot redeclare class PDO - It's entirely possible to have a version of PHP that pre-dates PDO, or specifically had PDO disabled at compile time.

KingCrunch Over a year ago

@Leigh Only, when the file is included, but I don't see a matching include anywhere.

lll Over a year ago

@KingCrunch: Makes sense, didn't include redefined class, didn't get dupe class error, and then calls a method from base class that also hasn't been defined.

Bruno Schäpper · Accepted Answer · 2012-07-30 06:48:43Z

I'd recommend to use autoloading. It would make your life simpler, and you could get rid of all those require calls.

There is an implementation in the PHP manual, I found it pretty useful. It might look something like this:

function my_autoloader($class) {
    include 'classes/' . $class . '.class.php';
}

spl_autoload_register('my_autoloader');

// Or, using an anonymous function as of PHP 5.3.0
spl_autoload_register(function ($class) {
    include 'classes/' . $class . '.class.php';
});

If you are on PHP 5.3, also consider using namespaces. It's actually even easier to have a good structure and a simple autoloader implemented by default:

spl_autoload_extensions(".php"); // comma-separated list
spl_autoload_register();

However, I remember a bug there. Usually I have a 'bootstrap' file, with this autoloader:

spl_autoload_register(function($class) {
    if(file_exists('./lib/' . str_replace('\\', '/', $class) . '.php')) {
        require_once './lib/' . str_replace('\\', '/', $class) . '.php';
    }
});

Using this mechanism, you have a folder structure that mimics your namespace structure. With this approach, you have a good structure, and do not need to care about including your libraries and other classes.

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