str_replace with ""

Does anyone know how I can use the php str_replace function for strings with " in them? for example “7” I would want to replace with “30”.

I was thinking:

$news = str_replace(" .\""7 . "\""," . \""30 . "\"",$news);

Not sure that will work.

Actually, what I really need is a way to change the sizes that each font size has for FCKeditor. When you change the size of the text, it says size=“7”, I want it to be 30.