How to include file & access class method from from within a method of another class?

Question

I have a class called numDisplay containing several methods. Most of those methods need to use a method from another class called prefs which is contained in another file during the course of their duties.

I think I could include the file, instantiate the class prefs and then access the method individually from inside each of the methods where it is needed, however, I expect there is a better way?

I thought of doing the file require_once in the __constructor of numDisplay and instantiate the class there also, then I could just call it from the methods of numDisplay .

I tried the code below and various syntax variations, but can't get it to work. How should I do this please?

class numDisplay {

    private $P;

    function __construct($P) {

        require_once($_SERVER['DOCUMENT_ROOT'].'/includes/classes/preferences.php');
        $P = new Preferences();

    }

    public function displayBSNo($num, $invType) {

        $auto_bs_no     = $this->$P->getPreference('auto_bs_no');
        $length_bs_no       = $this->$P->getPreference('length_bs_no');
        $length_alt_bs_no   = $this->$P->getPreference('length_alt_bs_no');

        if ($invType == 1) { // It's a standard BS

            if ($auto_bs_no == '1') {

                return "BS-" . $this->pad($num, $length_bs_no);

            } else {

                if ($num == '') {
                    return '<i>none</i>';
                } else {
                    return $num;
                }
            }

        }

        elseif ($invType == 2) {
            return "PI-" . $this->pad($num, $length_alt_bs_no);
        }
    }

}

When I attempt to use displayBSNo(), I get the following error:

Fatal error: Call to a member function getPreference() on a non-object in /home/peter/Documents/websites/Our_websites/bookkeeper.ph/books.bookkeeper.ph/public/includes/classes/common.classes.php on line 347

Answer 1

Instead of

$this->$P->getPreference('')

use

$this->P->getPreference('')

Answer 2

You're not setting the property in the constructor:

function __construct($P) {
    require_once($_SERVER['DOCUMENT_ROOT'].'/includes/classes/preferences.php');
    $P = new Preferences();
}

Here, $P is just a local variable in the scope of your constructor; to store the value inside the property P , you should have:

    $this->P = new Preferences();

In the rest of your code:

$this->$P->getPreference(...)

References the property of which the name is held in $P , which is probably not what you want. It should be:

$this->P->getPreference(...)

PHP actually emits notices when you make these kinds of mistakes; you can see them with this:

error_reporting(-1);
ini_set('display_errors', 'On');

Use these settings during development only.