Description:
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From manual page: http://php.net/manual/en/language.types.boolean.php#language.types.boolean.casting
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When converting to boolean, the following values are considered FALSE:

the boolean FALSE itself
the integer 0 (zero)
the float 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 empty tags <<<<<<<<<<<<< THIS!!!!

Every other value is considered TRUE (including any resource and NAN).
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The SimpleXML class is "magical" in that it overrides the general rule that any non-null object evaluates to true, and it has its own way of converting to boolean.

Yet there's no (documented and non-hacky) way a user-defined class can do the same and define its own rules to convert to boolean. There's no __toBoolean() (like there is a __toString()) magic method, and there's no operator overloading for overloading the == operator.

This is DISGUSTING. That a built-in class can do some magic that no user-defined class can do, is a symptom of an incredibly poor language design. The very moment you decided that non-null SimpleXML objects should evaluate to false in certain cases, you should have realized that any user-defined class could need to do the same thing, and you should have provided a generic mechanism of doing that, rather than hard-coding some magic into a particular built-in class.