Check if a property exists on magically set properties

Question

There is a lot of SO questions about the subject, notably this one, but it does not help me.

There is an ambiguity between property_exists and isset so before asking my question, I'm going to pointing it out:

property_exists

property_exists checks if an object contains a property without looking at its value, it only looks at its visibility.

So in the following example:

<?php
    
class testA
{
  private $a = null;
}
class testB extends testA
{
}

$test = new testA();
echo var_dump(property_exists($test, 'a')); // true

// parent's private property becomes invisible for its child

$test = new testB();
echo var_dump(property_exists($test, 'a')); // false

isset

isset checks if a value exists in a property, considering that is is not set if a value equals false and null.

<?php

$var = null;
echo var_dump(isset($var)); // false

$var = '';
echo var_dump(isset($var)); // true

$var = false;
echo var_dump(isset($var)); // true

$var = 0;
echo var_dump(isset($var)); // true

$var = '0';
echo var_dump(isset($var)); // true

isset and property_exists's behaviour on magically added properties

A property can exist with a null value, so I can't use __isset magic method to know if a property exist or not. I also can't use property_exists as properties are added using magic methods.

Here is a sample, but this is just a sample because in my app, properties magically set are stored outside the object.

class test {
    
    private $data = array();
    
    public function __get($key) {
        echo "get $key\n";
        return array_key_exists($key, $data) ? $data[$key] : null;
    }
    
    public function __set($key, $value) {
        echo "set $key = $value\n";
        $this->data[$key] = $value;
    }
    
    public function __isset($key) {
       echo sprintf("isset $key ( returns %b )", isset($this->data[$key]));
       return isset($this->data[$key]);
    }
    
}

$test = new test();
$test->x = 42;
isset($test->x); // 1

$test->y = null;
isset($test->y); // 0
property_exists($test, 'y'); // 0

So here is my question :

Is there a magic method or an SPL interface to implement property_exist with magically added properties ?

Donatello

Suggestion: use var_dump(), not echo sprintf.... – Chemesh

Also printf does exactly echo sprintf – Hungerford

Answer 1

I don't believe there's a way to alter the functionality of property_exists() using magic methods; here's a list of available magic methods in PHP. However, you should be able to alter isset() to use any logic you like.

class test {

    private $data = array();

    public function __get($key) {
        echo "get $key\n";
        return array_key_exists($key, $this->data) ? $this->data[$key] : null;
    }

    public function __set($key, $value) {
        echo "set $key = $value\n";
        $this->data[$key] = $value;
    }

    public function __isset($key) {
       echo sprintf("isset $key ( returns %b )", array_key_exists($key, $this->data));
       return array_key_exists($key, $this->data);
    }

}

$test = new test();
$test->x = 42;
isset($test->x); // 1

$test->y = null;
isset($test->y); // 1

This effectively fixes the (annoying) problem with isset and nulls by overriding its functionality through the magic method. Instead of using isset() within __isset() however, we use array_key_exists (which handles nulls as you would expect). Thus __isset() returns the expected result when a null value is set.

This has a downside, namely that the overridden functionality does not produce the same results as default isset() functionality. So, if this object needs to be used transparently with other (perhaps stdClass) objects, then isset() will return true for null values in objects of this class, and false for null values in normal objects.

Depending on your needs then this may or may not be a viable solution. If the above issue is a hindrance, then another option might be to define an interface with a keyIsSet() property, and apply that interface to all objects to be tested. Then use $obj->keyIsSet('key') rather than isset($obj->$key). Not as elegant, but a bit better oo.

Bloomfield

Simple, I don't know how I could not have thought about array_key_exists before. Thanks. – Donatello

Ok now I remember, when I needed this, my properties were stored OUTSIDE the current class (on a parent's one), so it was not possible to use an array to store them, as __get should haven't been called as the property really existed on the final object. Anyway, this is an old question so I leave your answer accpeted. – Donatello

@AlainTiemblo It doesn't matter where you store the properties. property_exists() will return true if a property is set on a parent class. The issue occurs when you don't set the magic property where property_exists() will find it, as it cannot be altered. See my answer to this. – Cypro

Answer 2

Problem is with the implementation of __set and __get functions, I modified them and is works for both isset and property_exists

<?php

class test {

    private $data = array();

    public function __get($key) {
        echo "get $key\n";
        return $$key;
    }

    public function __set($key, $value) {
        echo "set $key = $value\n";
        $this->$key = $value;
    }

    public function __isset($key) {
       echo sprintf("isset $key ( returns %b )", isset($this->$key));
       return isset($this->$key);
    }

}

$test = new test();
var_dump(property_exists($test, 'x'));
var_dump(isset($test->x));

$test->x = 42;

var_dump(property_exists($test, 'x'));
var_dump(isset($test->x));
?>

Wilmawilmar

As mentionned, properties magically set are stored outside the object. Thanks anyway. – Donatello