note 24805 deleted from language.types.boolean by nicos

Remember that TRUE and FALSE are no more case sensitive in PHP4. In PHP3 they were predefined constants with the value true and false. So: TRUE === true === True === TrUe FALSE === false === False === FaLsE also: TRUE === (boolean)1 === (boolean)2 FALSE === (boolean)0 === (boolean)"" === (boolean)NULL === (boolean)'' and: TRUE == 1 but TRUE !== 1 FALSE == 0 but FALSE !== 0 and: TRUE === !FALSE === !0 FALSE === !TRUE === !1 === !-1 and: TRUE === empty(FALSE) === empty(0) === empty(0.0) === empty('') === empty(' ') === empty('0') === empty(' 0') === empty('0.0e0') FALSE === empty(TRUE) === empty(1) === empty(-1) === empty(24) === empty(-12.0) === empty('17.0e0') but: TRUE === empty('A1') === empty(NULL) === empty($unset_variable) FALSE === empty('0 ') === empty('00') Globally, the empty() built-in function is identical to the ! operator, except that it avoids a warning on the unset variable; in any case, empty(expression) === @!(expression). The following will generate warnings: if (!$unset_variable) { // generates a warning, but this branch is taken } else { // branch not taken } This similar code generates no warning: if (empty($unset_variable)) { // this branch is taken silently } else { // branch not taken } So you can convert to a boolean any variable silently with this: !empty($variable) // no warning if not set instead of: (boolean)$variable // warning if not set!