You can have either one replacement for all words or a separate replacement for each word. But if you only want one replacement I think it might have to be a string instead of an array, like:
PHP Code:
$censoredword = array(
'swear word1',
'swear word2'
);
$changedword = 'new word for all swears';
$this->post['message'] = str_ireplace($censoredword, $changedword, $this->post['message']);