Description:
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Two proposed PDO fetch combinations I think would be sensible (I've actually tried to use these on occasion, assuming they would work).
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1. PDO::FETCH_GROUP | PDO::FETCH_KEY_PAIR
Same as FETCH_KEY_PAIR but organised into groups. The first column would be the group, the second the key and the third the value. E.g.:
[
  'group1'=>[
    'key1'=>'val1',
    'key2'=>'val2'
  ],
  'group2'=>[
    ...
  ]
]
Currently this combination appears to ignore FETCH_GROUP and gives identical results as FETCH_KEY_PAIR by itself, despite the documentation for FETCH_GROUP indicating that it can be combined with FETCH_KEY_PAIR. See also https://bugs.php.net/bug.php?id=63813.
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2. PDO::FETCH_GROUP | PDO::FETCH_UNIQUE
Same as FETCH_UNIQUE but organised into groups. The first column would be the group, the second the unique key. E.g.:
[
  'group1'=>[
    'key1'=>['col3'=>'val', 'col4'=>'val', ...],
    'key2'=>['col3'=>'val', 'col4'=>'val', ...]
  ],
  'group2'=>[
    ...
  ]
]
Currently this combination appears to ignore FETCH_GROUP and gives identical results as FETCH_UNIQUE by itself, as PDO::FETCH_GROUP|PDO::FETCH_UNIQUE === PDO::FETCH_UNIQUE.
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There may be BC issues with implementing these if people are currently using these combinations believing they are achieving something (https://stackoverflow.com/a/22063422).