What is the efficient way to create an array of associative arrays in PHP?

Perhaps something like this?

$i = 0;
foreach($resultSet as $key => $value){
    $output[$i++][$key] = $value;
}

Given a $resultSet like this:

$resultSet['key0'] = 'value0';
$resultSet['key1'] = 'value1';
$resultSet['key2'] = 'value2';
$resultSet['key3'] = 'value3';
$resultSet['key4'] = 'value4';
$resultSet['key5'] = 'value5';

The above code will produce something as follows:

Array (
    [0] => Array ( [key0] => value0 )
    [1] => Array ( [key1] => value1 )
    [2] => Array ( [key2] => value2 )
    [3] => Array ( [key3] => value3 )
    [4] => Array ( [key4] => value4 )
    [5] => Array ( [key5] => value5 )
)

Echoing $output[0]['key0] will output value0 (or an array if value0 is an array).

As a side note, you could actually get completely rid of $i, and just do the following:

foreach($resultSet as $key => $value){
    $output[][$key] = $value;
}

Do let me know if I completely misunderstood the question at hand though :)

Instead of declaring $i=0; before the loop, you can also write :

foreach($resultSet as $i => $r)
{
  $output[$i]['key1'] = $r['col1'];
  $output[$i]['key2'] = $r['col2'];
  ...
  $output[$i]['keyN'] = $r['colN'];
}

Also, if you could rename your column in the DB query, you could have :

foreach($resultSet as $i => $r)
{
  $output[$i] = $r;
}

Which is pretty much like doing $output = $resultSet;

Edit : If you do need to do some processing on $resultSet for whatever reason and you'd rather have different names, you can consider doing :

for (array('key1' => 'col1', ... 'keyN' => 'colN' as key => col)
{
   $output[$i][$key] = $r[$col];
}