Make certain characters of a word from string bold

PHP Snippet 1:

$text = preg_replace('/\S*('. $searhPhrase .')\S*/i', '<b>$1</b>', $text);

PHP Snippet 2:

$text = preg_replace('/(\S*'. $searhPhrase .'\S*)/i', '<b>$1</b>', $text);

PHP Snippet 3:

function strReplace($searchText,$mainText){
    return str_replace($searchText, '<b>'.$searchText.'</b>', $mainText);
  }