Just read the manual, it says:

When converting to bool, the following values are considered false:

• the boolean false itself
• the integer 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 attributeless empty elements, i.e. elements which have neither children nor attributes.

Every other value is considered true (including any resource and NAN).