I am trying to create a simple BB code like script in PHP for my forum. The problem is, I am not sure how to stop a user posting something like this..
[b] Hahah
[b] Will make all of your text
[b] on this page bold
This would display (because I'm replacing words)
<strong> Hahah
<strong> Will make all of your text
<strong> on this page bold
What would be a good way to prevent this and end the tags at the bottom of the post?
Replacing BBCode by HTML is actually not as easy as it seems. Just using str_replace() will never be good enough. Because, as you said, people can just only place the open tag and this way, change your layout. A popular 'solution' is to use preg_replace() instead, with a regular expression that matches both the open and close tag. That, however, is quite unsafe with BBCode tags like [url] (you can insert JavaScript in those URL tags).
However, there are some libraries out there that are very good in replacing BBCode. StringParser_BBCode is one of those. Those libraries do a little more than just replacing. For example, they will usually prevent malformed BBCode like [b][i]foo[/b][i] to turn into <strong><em>foo</strong></em> (which is malformed HTML). And also, they can do a lot more on preventing XSS injections.