Flag echo usage errors when selected(), disabled(), checked() helpers

Consider:

$output .= selected( in_array( $category->slug, $selected_options ) );

This will not have the desired effect, since the 3rd argument to selected(), $echo, defaults to true.

It would be helpful if there was a sniff that looked at any instance of selected(), disabled(), checked(), etc that have an lvalue (assigned to a variable) if they do not supply the 3rd argument to the function, as such instances are surely accidental omissions of that argument.

The above PHP code could be automatically fixed up to be:

$output .= selected( in_array( $category->slug, $selected_options ), true, false );

Start with the PHP example and locate the WordPress Coding Standards sniffs for selected(), disabled(), and checked(). Check how an assignment is recognized and whether the third $echo argument is omitted; done means those cases are flagged and the shown false-argument correction is supported.