str_replace for postbit its posible?

[WarLion]

i have one question is possible to make a str_replace i will trying from 4 hours now and i still cant do it
its posible?
i try with diferent hooks but not luck.
this code is what i need

$vbulletin->templatecache['postbit'] = str_replace('<img src=\"$post[avatarurl]\"', '<span class="test"> <img src=\"$post[avatarurl]\"', $vbulletin->templatecache['postbit']);
$vbulletin->templatecache['postbit'] = str_replace('$vbphrase[xs_avatar]</phrase>" border=\"0\" />', '$vbphrase[xs_avatar]</phrase>" border=\"0\" /></span>', $vbulletin->templatecache['postbit']);