PHP 7.4 is EOL and I masked it yesterday, but grknight pointed out that exact-image still depends on it with USE=php. I see that there are no new releases of exact-image, but maybe it works with newer php-8.x releases without any changes? As mentioned in bug 780864 this is probably more of a swig issue. I'll add a package.use.mask for it in the meantime, but eventually if it cannot work with modern versions of PHP then the USE flag will probably have to be dropped.
php USE flag was removed