Url

[NZ_Kiwis]

I have a page which will have a URL like this
posts.php?thread_id=42&main_id=5
but of course the id's could be anything

I want a code to see if the URL is just posts.php
and if it is to redirect them...

[sissy]

you could use

$_SERVER['QUERY_STRING'];

test and redirect off that

[cpuworks]

Or, if you mean the values for the IDs could be anything, but those IDs will be set normally to something, you could test for them:

if (! isset($_REQUEST['main_id']) && ! isset($_REQUEST['thread_id'])) {
   // neither one sent
} else {
   // etc.
}