If you want something elegant in your code, when dealing with reducing array, just unshift first element, and use it as initial, because if you do not do so, you will + first element with first element:
<?php
$arr = array(
array('min' => 1.5456, 'max' => 2.28548, 'volume' => 23.152),
array('min' => 1.5457, 'max' => 2.28549, 'volume' => 23.152),
array('min' => 1.5458, 'max' => 2.28550, 'volume' => 23.152),
array('min' => 1.5459, 'max' => 2.28551, 'volume' => 23.152),
array('min' => 1.5460, 'max' => 2.28552, 'volume' => 23.152),
);
$initial = array_shift($arr);
$t = array_reduce($arr, function($result, $item) {
$result['min'] = min($result['min'], $item['min']);
$result['max'] = max($result['max'], $item['max']);
$result['volume'] += $item['volume'];
return $result;
}, $initial);
?>
