What is this !! operator in PHP?

Question

I was analyzing a code in PHP and came across the following:

$result = !! $row;

return $result;

What is this !! and what he does?

Answer 1

if uses 2 !! to convert to binary Type-safe, or if the method or function that uses this value makes a gettype the result will be Boolean. Not all API or lib rely on dynamic typing maybe this is the case of use.

When Converting to Boolean, the following values are considered FALSE:

the Boolean FALSE itself

the integers 0 and -0 (zero)

the floats 0.0 and -0.0 (zero)

the Empty string, and the string "0"

an array with zero Elements

the special type NULL (including unset variables)

Simplexml Objects created from Empty tags

and in the case (bool)$row and !!$row generate the same result bool(true|false).

@from: https://www.php.net/manual/en/language.types.boolean.php

Answer 2

Complementing the Jroger answer, and applying in the context of the question code, the double use of the negation operator !! can be used to find out if an array is empty or not, as a replacement for count($array) > 0 or !(empty($array)).

<?php
  $a = !![];   // retorna false;

  echo ($a ? 'array cheia' : 'array vazia'). PHP_EOL; // retorna 'array vazia'

  $array = ["um", "dois", "três"];

  echo (!!$array ? 'array cheia' : 'array vazia') . PHP_EOL; // retorna 'array cheia'
?>

Can also be used with string (not advisable because the string "0" gives false result to empty):

<?php

  echo (!!'testing' ? 'string cheia' : 'string vazia') . PHP_EOL; // retorna 'string cheia'

  echo (!!'' ? 'string cheia' : 'string vazia') . PHP_EOL; // retorna 'string vazia'

?>