PHP OOP issue with dynamic multidimensional arrays

Question

I have an issue with the following piece of code:

<?php
    class testClass
    {
        public $settings;

        public function __construct()
        {
            $this->settings = array(
                'paths' => array(
                    'protocol' => 'http'
                )
            );
        }

        public function getSomething()
        {
            $string = "settings['paths']['protocol']";

            echo $this->{$string};      /***** Line 19 *****/
        }
    }


    $obj = new testClass;
    $obj->getSomething();                          // Outputs a 'undefined' notice
    echo '<br />';
    echo $obj->settings['paths']['protocol'];      // Outputs http as expected
?>

This is a very basic example of the code I am employing, the actual code is more advanced, but the output / error produced is the same.

Basially, the class constructor populates a property with a settings array. The getSomething() method assigns an array path to a variable, which is then attempted to be retrieved by the echo $this->{$string}; code.

When I write: $obj->getSomething(); I get the following error:

Notice: Undefined property: testClass::$settings['paths']['protocol'] in /test.php on line 19

If I write the following code echo $obj->settings['paths']['protocol'] I get the expected http

I'm not sure why this isn't working!! If anyone can shed any light, it would be greatly appreciated.

Thanks

deceze · Accepted Answer · 2013-03-27 13:35:03Z

2

Well, you don't have a property called "settings['paths']['protocol']". You have a property called settings which has the key paths which has the key protocol. But PHP doesn't interpret $this->{$string} like copy and pasting code, it looks for a property called "settings['paths']['protocol']", which doesn't exist. That's nothing particular to OOP code, it's how any variable variable works.

I'd suggest something like this instead:

/**
 * Get settings, optionally filtered by path.
 *
 * @param string $path A path to a nested setting to be returned directly.
 * @return mixed The requested setting, or all settings if $path is null,
 *               or null if the path doesn't exist.
 */
public function get($path = null) {
    $value = $this->settings;

    foreach (array_filter(explode('.', $path)) as $key) {
        if (!is_array($value) || !isset($value[$key])) {
            return null;
        }
        $value = $value[$key];
    }

    return $value;
}

Called like this:

$obj->get('paths.protocol');

And just for fun, here a functional implementation of the above: ;-3

public function get($path = null) {
    return array_reduce(
        array_filter(explode('.', $path)),
        function ($value, $key) { return is_array($value) && isset($value[$key]) ? $value[$key] : null; },
        $this->settings
    );
}

edited Mar 27, 2013 at 13:35

answered Mar 27, 2013 at 13:12

deceze♦

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3 Comments

Phil Cross Over a year ago

Do you know of any workarounds for this type of issue? Thanks for the explanation, I understand the problem now!

deceze Over a year ago

What exactly do you want to do? The example code hardly makes sense, since you wouldn't actually use it like that, I suppose.

Phil Cross Over a year ago

Many thanks for the code fix! Its worked, and saved me much stress! Many thanks again.

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