This is the easiest type. A boolean expresses a truth value. It can be either TRUE or FALSE.
Note: The boolean type was introduced in PHP 4.
To specify a boolean literal, use either the keyword TRUE or FALSE. Both are case-insensitive.
To explicitly convert a value to boolean, use either the (bool) or the (boolean) cast. However, in most cases you do not need to use the cast, since a value will be automatically converted if an operator, function or control structure requires a boolean argument.
See also Type Juggling.
When converting to boolean, the following values are considered FALSE:
the boolean FALSE itself
the integer 0 (zero)
the float 0.0 (zero)
an array with zero elements
an object with zero member variables
the special type NULL (including unset variables)
-1 is considered TRUE, like any other non-zero (whether negative or positive) number!