Falsy

A falsy value is a value that evaluate to false, when cast to a boolean. It includes false, obviously, but also other values such as 0, '', the empty string, or [], the empty array.

The contrary to falsy is truthy.

There are some falsy values which are difficult to guess, as is illustrated below.

<?php

var_dump((bool) 0);       // false
var_dump((bool) '');      // false
var_dump((bool) '0');     // false
var_dump((bool) '00');    // true
var_dump((bool) []);      // false
var_dump((bool) [null]);  // true
var_dump((bool) null);    // false

?>

See also Truthy and Falsy in PHP and PHP Tricky True False Examples.

Related : Truthy, False, Boolean, Empty String