Description:
------------
The imagetypes function will not return a bitmask value for if WEBP support is available, which is available with read/write since PHP 5.5. All other formats are currently covered (except for GD which is implied, and XBM which is implied by XPM).

Here's the code from PHP7 to confirm...

/* {{{ proto int imagetypes(void)
   Return the types of images supported in a bitfield - 1=GIF, 2=JPEG, 4=PNG, 8=WBMP, 16=XPM */
PHP_FUNCTION(imagetypes)
{
	int ret=0;
	ret = 1;
#ifdef HAVE_GD_JPG
	ret |= 2;
#endif
#ifdef HAVE_GD_PNG
	ret |= 4;
#endif
	ret |= 8;
#if defined(HAVE_GD_XPM)
	ret |= 16;
#endif

	if (zend_parse_parameters_none() == FAILURE) {
		return;
	}

	RETURN_LONG(ret);
}


Personally I'd recommend just dropping the function, given that GD-Info returns details in its report and you can just use function_exists. Removing a function and a set of constants may lower the PHP overhead slightly. But if it stays it should be consistent with what PHP can do.