htaccess rewrite url and check for 404 - php

Question

I`ve got an internet shop and want to use htaccess to shorten links
there are 3 cases of urls:
shop.com/shop/18 (number) - products.php?categoryid=$1
shop.com/shop/18/page-2 (number)/(page+number) - products.php?categoryid=$1&page=$2
shop.com/shop/18/9877 (number)/(number) - description?categoryid=$1&productid=$2
my try
RewriteRule ^shop/?$ shop.php
RewriteRule ^shop/(.*)/([0-9]+)/?$ description.php?categoryid=$1&productid=$2
RewriteRule ^shop/(.*)/page-(.*)/?$ products.php?categoryid=$1&page=$2
RewriteRule ^shop/(.*)/?$ products.php?categoryid=$1
With my try - 1 (works), 2 (works), 3 (doesn`t work)
How can I rewrite urls so?
How can I redirect to 404 page if e.g. there is no such number of
category or such product (guess check with php and mysql and then
redirect) ?

Answer 1

There are a number of ways that this can be dealt with;
All in htaccess (gets messy with multiple depths)
Combined htaccess and server side code
The best approach is the one that suits you based on how your store is coded. I personally feel that handling it in the server side code is better, it simplifies the htaccess file, and gives you more control with regards to validating data, and how you handle what is sent, to where, and how its processed when it gets there.
For example, in my htaccess file I have;
<IfModule mod_rewrite.c>
Options +FollowSymlinks
RewriteEngine on
#
# Do not apply rewrite rules for non required areas
RewriteCond %{REQUEST_URI} "/hidden-areas/" [OR]
RewriteCond %{REQUEST_URI} "/other-areas/"
RewriteRule (.*) $1 [L]
# Do Not apply if a specific file or folder exists
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
# The rules on how to rewrite the urls
RewriteRule (.*) /index.php?url=$1 [QSA,L]
</IfModule>
Basically, to explain this in a nutshell, I DONT rewrite anything for certain folders, I forward them straight on. This is to stop calls to scripts externally, or extra added systems being able to be accessed without issue.
I then, forward the entire url as a string through to my index page, and deal with what comes through using PHP, an example is below.
// collect the passed url
$url = $_GET['url'];
// split the url into parts
$url_parts = explode('/', $url);
/*
* start sorting what is what in the url
*/
// count how many parts there are
$url_parts_count = count($url_parts);
// determine the class/module
$class = $url_parts[0]; // generally the class/method/module depending on your system, thgough could be a category so run some checks
// determine the last part in the array
$last_url_part = ($url_parts_count - 1);
// set the last part of the url to be used
$slug = $url_parts[$last_url_part]; // generally the slug and will be empty if theres a trailing slash
etc etc etc
This is just a summary, i do far more, as this is taken from a CMS I wrote, but it should give you a very good starting point should you wish to get your hands dirty. Of course, Im happy to elaborate further if necessary.
The caveat of course, is if you are using an off-the-shelf system, they should provide you with this code already ;)
I have added below something based on your updated question, this will help if you do still plan to go the way you are :)
<IfModule mod_rewrite.c>
Options +FollowSymlinks
RewriteEngine on
RewriteBase /
#
# Do not apply rewrite rules for non required areas
RewriteCond %{REQUEST_URI} "/hidden-areas/" [OR]
RewriteCond %{REQUEST_URI} "/other-areas/"
RewriteRule (.*) $1 [L]
# Do Not apply if a specific file or folder exists
# RewriteCond %{REQUEST_FILENAME} !-f
# RewriteCond %{REQUEST_FILENAME} !-d
# The rules on how to rewrite the urls
RewriteRule ^([a-zA-Z0-9_-]+)$ /index.php?slug=$1 [QSA,L]
RewriteRule ^([a-zA-Z0-9_-]+)/$ /index.php?type=$1 [QSA,L]
RewriteRule ^([a-zA-Z0-9_-]+)/([a-zA-Z0-9_-]+)$ /index.php?type=$1&slug=$2 [QSA,L]
RewriteRule ^([a-zA-Z0-9_-]+)/([a-zA-Z0-9_-]+)/$ /index.php?type=$1&cat=$2 [QSA,L]
RewriteRule ^([a-zA-Z0-9_-]+)/([a-zA-Z0-9_-]+)/([a-zA-Z0-9_-]+)$ /index.php?type=$1&cat=$2&slug=$3 [QSA,L]
</IfModule>

Answer 2

Thanks, ended with
RewriteRule ^shop$ shop.php [L]
RewriteRule ^shop/([0-9]+)$ products.php?categoryid=$1 [L]
RewriteRule ^shop/([0-9]+)/(page-[0-9]+)$ products.php?categoryid=$1&page=$2 [L]
RewriteRule ^shop/([0-9]+)/([0-9]+)$ description.php?categoryid=$1&productid=$2 [L]