In the past when you forget to add a PHP extension in FreeBSD you have to recompile the whole thing, but now PHP port support dynamic extensions, in fact PHP port doesn’t allow you to add any extensions, you have to add these extensions using a separate port.
There is one wrapper port called /usr/ports/lang/php4-extensions/ which will allow you to select modules to install.
Very nice indeed, even when I spent long time to figure out how to add extensions.