Sniff for detecting single-use variables?

[lkraav]

Trying to avoid this example BAD (due to refactoring, or simple human error):

$has_free_tag = has_tag( 'free' );

if ( $has_free_tag ) {
  ...
}

BETTER:

if ( has_tag( 'free' ) ) {
  ...
}

[sirbrillig]

That would be pretty cool! And not too hard, I think.