<?php

class Foo {
    public $prop = null;
}

class Bar {
    public function __construct() {
        $this->prop = null;
    }
}

var_dump(property_exists('Foo', 'prop'));
var_dump(property_exists('Bar', 'prop'));

$foo = new Foo;
$bar = new Bar;

var_dump(property_exists($foo, 'prop'));
var_dump(property_exists($bar, 'prop'));