<?php
$string = '<a href="hoge">ここ</a> ここ <a href="hoge">ほげ</a>';
$patterns = '/(<a.*?<\/a>)|(ここ)/e';
$replace = '"\1"?"\1":"<strong>\2</strong>"';
?>
PD9waHAKJHN0cmluZyA9ICc8YSBocmVmPSJob2dlIj7jgZPjgZM8L2E+IOOBk+OBkyA8YSBocmVmPSJob2dlIj7jgbvjgZI8L2E+JzsKJHBhdHRlcm5zID0gJy8oPGEuKj88XC9hPil8KOOBk+OBkykvZSc7CiRyZXBsYWNlID0gJyJcMSI/IlwxIjoiPHN0cm9uZz5cMjwvc3Ryb25nPiInOwplY2hvIHByZWdfcmVwbGFjZSgkcGF0dGVybnMsICRyZXBsYWNlLCAkc3RyaW5nKTsKPz4=