How to rewrite this URL:
https://example.com/illustrations.php?category=cats&cat_id=1 to:
https://example.com/category/cats also, how do I still preserve cat_id param?
I tried this but it does not work:
RewriteEngine on RewriteCond %{REQUEST_FILENAME} !-f RewriteCond %{REQUEST_FILENAME} !-d RewriteRule ^/?category/(.*?)/?$ /illustrations.php?category=$1 [L] When I add this code and go to
illustrations.php?category=catsit does not change the URL in the browserbar.
Yes, that is "correct".
The code you posted internally rewrites the URL /category/cats (which is the URL you should be linking to in your HTML source) back to the actual filesystem/URL path: /illustrations.php?category=cats. This is required in order to make the "pretty" URL /category/cats "work".
You can't change the URL structure using .htaccess only - if that is what you are implying. You do need to actually change the physical URLs in your HTML source.
You could implement an external redirect (not a rewrite) from /illustrations.php?category=cats to the canonical URL /category/cats using .htaccess, but note that this is only to benefit SEO (and third parties that might have already linked to the old URLs). This is a necessary additional step if you are changing an existing URL structure and SEO is a concern, but it is not part of the "working" of your site.
how do I still preserve
cat_idparam
You would need to include the value of the cat_id parameter in the URL. eg. /category/cats/1 (as @arkascha suggested in comments) or /category/1/cats - depending on which value is more important. I would put the more important value first, since URLs can be accidentally cropped when shared.
RewriteEngine on RewriteCond %{REQUEST_FILENAME} !-f RewriteCond %{REQUEST_FILENAME} !-d RewriteRule ^/?category/(.*?)/?$ /illustrations.php?category=$1 [L]
This rule could be simplified. Filesystem checks are relatively expensive. It looks like you could remove both of these by making your regex more specific. eg. Could /category/1/cats ever map to a file? Do you need directories to be accessible? I would expect the answer to both those questions is "no".
I would also decide on whether to allow trailing slashes or not, rather than allowing both (as in your current rule). Strictly speaking this creates duplicate content (two URLs; same content), so requires additional steps to resolve. Your example URL(s) do not contain a trailing slash.
So, you could simplify your rules to the following in order to rewrite /category/1/cats (no trailing slash) to /illustrations.php?category=cats&cat_id=1
RewriteEngine On RewriteRule ^category/(\d+)/([\w-]+)$ /illustrations.php?category=$2&cat_id=$1 [L] This assumes cat_id can be 1 or more digits (0-9). And category is limited to the following characters: a-z, A-Z, 0-9, _ (underscore), - (hyphen).
With regex it is preferable to be as restrictive as possible. By omitting the dot (.) from the last path segment in the above rule it cannot match a physical file (assuming all your files have file extensions).
/category/catsthen the only thing you can do is to seemingly hard-code thecat_idparameter in the substitution string. Otherwise, you need to include the cat_id in the URL. "How to rewrite this URL..." - it's the other way round, you are rewriting that URL to this URL (which is what you are doing in your.htaccessrule).https://example.com/category/cats/1. Without that there is no way to implement a working redirection since the rewriting module cannot somehow make up such ids by magic.