regex - PHP preg_replace: Case insensitive match with case sensitive replacement

Question

I'm using preg_replace in PHP to find and replace specific words in a string, like this:

$subject = "Apple apple";
print preg_replace('/apple/i', 'pear', $subject);

Which gives the result 'pear pear'.

What I'd like to be able to do is to match a word in a case insensitive way, but respect it's case when it is replaced - giving the result 'Pear pear'.

The following works, but seems a little long winded to me:

$pattern = array('/Apple/', '/apple/');
$replacement = array('Pear', 'pear');
$subject = "Apple apple";
print preg_replace($pattern, $replacement, $subject);

Is there a better way to do this?

Update: Further to an excellent query raised below, for the purposes of this task I only want to respect 'title case' - so whether or not the first letter of a word is a capital.

Answer 1

I have in mind this implementation for common case:

$data    = 'this is appLe and ApPle';
$search  = 'apple';
$replace = 'pear';

$data = preg_replace_callback('/'.$search.'/i', function($matches) use ($replace)
{
   $i=0;
   return join('', array_map(function($char) use ($matches, &$i)
   {
      return ctype_lower($matches[0][$i++])?strtolower($char):strtoupper($char);
   }, str_split($replace)));
}, $data);

//var_dump($data); //"this is peaR and PeAr"

-it's more complicated, of course, but fit original request for any position. If you're looking for only first letter, this could be an overkill (see @Jon's answer then)