5.php

<?php

error_reporting(E_ALL|E_STRICT);
ini_set('display_errors', 1);

class T {
    public $dupa = 1;
}

$t = new T;
echo $t->dupa . PHP_EOL;
echo (isset($t->dupa) ? 'set' : 'non set') . PHP_EOL;

$t->dupa = '';
echo (isset($t->dupa) ? 'set' : 'non set') . PHP_EOL;

$t->dupa = 0;
echo (isset($t->dupa) ? 'set' : 'non set') . PHP_EOL;

$t->dupa = null;
echo (isset($t->dupa) ? 'set' : 'non set') . PHP_EOL;

$t->dupa = false;
echo (isset($t->dupa) ? 'set' : 'non set') . PHP_EOL;

#    1 1
#    2 set
#    3 set
#    4 set
#    5 non set
#    6 set