Eseguire sostituzione di parte di una stringa mantenendo le maiuscole

[boots]

io farei una cosa del genere:

$string = "qwerAbCfg";
$search ="/abc/i";
$matches = array();
preg_match_all($search, $string, $matches, PREG_OFFSET_CAPTURE);

$output = ''; 
$start = 0;
foreach($matches[0] as $match){
    $output .= substr($string, $start, $match[1] - $start);
    $start += $match[1] - $start ;
    $output .= '<b>'.substr($string, $match[1], strlen($match[0]) ).'</b>';
    $start += strlen($match[0]);
 }
$output .= substr($string, $start);
echo $output;