对于普通情况，我已经想到了此实现:

I have in mind this implementation for common case:

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

$data = preg_replace_callback('/\b'.$search.'\b/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"

-当然更复杂，但是适合任何职位的原始要求.如果您只寻找第一个字母，这可能是一个过大的选择(请参阅@Jon的回答)

-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)