I want to modify a Mysql query string
in PHP as follows. Can you please correct me?

$query = str_replace("WHERE", "WHERE prospectid > \"$prospectID\" AND", $query);

That's insufficient information. Is the prospectid field a number? If so, you're treating it like a string. If not, you'll need to provide a bit more so that we can understand your purpose and what the code is not doing that you think it ought.

Thanks for trying to make your code stand out. You can also use [php] tags to properly delineate PHP code.