Truthy

A truthy value is a value that evaluate to true, when cast to a boolean. It includes true, obviously, but also other values such as 1, -1, '0', strings, except the empty string and '0, non-empty arrays and all the objects.

The contrary to truthy is falsy.

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

<?php

var_dump((bool) 1);       // true
var_dump((bool) -1);      // true
var_dump((bool) '0');     // false
var_dump((bool) '00');    // true
var_dump((bool) []);      // false
var_dump((bool) [null]);  // true
var_dump((bool) new stdclass);    // true

?>

See also The Differences in Truthiness and Falsiness in JavaScript vs PHP.

Related : False, Falsy, True, Boolean, Comparison, one, Minus One -1, zero