php - Unexpected output on a ANDING boolean expression in a class and a single if statement -

i wrote function getting data out of array. part in function verification of data, checks if key not empty , if array key not exists (my bad after edit). function looks this:

public function getdata($key = "") {     if (!empty($key) && !array_key_exists($key, $this->data)) {         throw new configurationexception("$key not exist");     }     return empty($key) ? $this->data : $this->data[$key]; } 

after applying srp princple, putting 'verification' class, looked this:

class configurationverificationhandler {      public function ispresent($key, array $data)     {         return empty($key) && array_key_exists($key, $data);     } }  public function getdata($key = "") {     $verificationhandler = new configurationverificationhandler();     if (!$verificationhandler->ispresent($key, $this->data)) {         throw new configurationexception("$key not exist");     }     return empty($key) ? $this->data : $this->data[$key]; } 

from both of these snippets, expected same result, because put ! operator before ispresent function, making check if condition false.

how come boolean expressions not same , dont give same result?

!a && !b not same !(a && b). 1 has @ case 1 variable true (let's pick a) , other false:

!true && !false --> false && true --> false  !(true && false) --> !(false) --> true 

however, !a && !b is equivalent !(a || b):

!(true || false) --> !(true) --> false