Zval type flags are incorrect for IS_CALLABLE / IS_INDIRECT

[joehoyle]

While trying to working with ExecutorGlobals::get().symbol_table where the PHP global variables can be INDIRECT Zvals. (See https://wiki.php.net/phpng-int for a description of INDIRECT Zvals, specifically around their use in symbol tables.

ext-php-rs is detecting the type of INDIRECT zval's as Callable, which is not correct. Looking at the PHP src at https://github.com/php/php-src/blob/master/Zend/zend_types.h#L562-L575 there is overlap in IS_CALLABLE and IS_INDIRECT (similarly IS_PTR and IS_ITERABLE). For some reason ext-php-rs has chosen to take the flag IS_CALLABLE in https://github.com/davidcole1340/ext-php-rs/blob/master/src/flags.rs#L254, when it should actually be checking IS_INDIRECT and treating it like a pointer to another Zval.

I don't believe Zval's type flag can actually be IS_CALLABLE, as is suggested by the php-src link above, that is a "fake" type.

Switching this out would probably break the current implementation of Zval::is_callable() though, so I'm not sure what the correct solution here is exactly.