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<?php $a = []; $a['b']['c']['d'] = 1; $a['b']['c']['d2'] = new stdclass(); $a['b']['c']['d3'] = []; var_dump(isset($a['b']['c']['d'])); // as expected, and within type var_dump(isset($a['b']['c']['f'])); // as expected, and within type var_dump(isset($a['b']['c']['d']['g'])); // as expected, because 'd' is 1 var_dump(isset($a['b']['c']['d']->g)); // as expected, because 'd' is 1 var_dump(isset($a['b']['c']['d3']->f)); // as expected, because 'd' is an array var_dump(isset($a['b']['c']['d3']['f'])); // as expected, because 'd' is an array var_dump(isset($a['b']['c']['d2']->f)); // as expected, because 'd' is an object var_dump(isset($a['b']['c']['d2']['f'])); // fatal error!! var_dump(isset($a['b']['c']['D']['g']->f)); // as expected, because 'D' is actually null var_dump(isset($a['b']['c']['D']['g']['f'])); // as expected, because 'D' is actually null // isset($a['b']['c']['d']) is set so following are true and within type var_dump(isset($a['b']['c'])); var_dump(isset($a['b'])); $i = 1; echo $i[3]; // error,