$results = array(
    array(
        'id' => 0,
        'key' => 'somekey',
        'value' => 'This is a string.'
    ),
    array(
        'id' => 0,
        'key' => 'someotherkey',
        'value' => 100002
    ),
    array(
        'id' => 1,
        'key' => 'someotherkey',
        'value' => 100002
    ),
    array(
        'id' => 2,
        'key' => 'somekey',
        'value' => 'This is another string'
    ),
    array(
        'id' => 2,
        'key' => 'someotherkey',
        'value' => 100002
    ),
); //This is like what you get from MySQL.  Each array is a result from the database.

$crosstab = array();

foreach($results as $result) {
    if(!is_array($crosstab[$result['key']]) {
        $crosstab[$result['key']] = array();
    }
    $crosstab[$result['key']][$result[id]] = $result['value'];
}

var_dump($crosstab);