htaccess rewrite and redirection for SEO friendly URLs

Question

I know that this question may be duplicate or answered in the past but I have read the apache.org site and have seen many other similar answers and I cannot find a proper solution to my problem so far since I am novice in the .htaccess and apache.

For simplicity reasons let's assume that the URL of my site is: https://www.example.com

The main page is index.php and there is a submit form with 3 select/dropdowns elements with names: sp, ar and in. sp is mandatory (user has to enter a value), ar and in are optionals.

When I submit the form, the file results.php is executed. When all 3 dropdowns have values, the form of the submitted URL is:

1) https://www.example.com/results.php?sp=textvalue1&ar=textvalue2&in=textvalue3

Moreover, depending if the ar and in have (or haven't) values, the other possible forms of the URL are:

2) https://www.example.com/results.php?sp=textvalue1&ar=&in= 
3) https://www.example.com/results.php?sp=textvalue1&ar=textvalue2&in= 
4) https://www.example.com/results.php?sp=textvalue1&ar=&in=textvalue3 

I want to convert these "ugly" URLs to SEO-friendly URLs.

For case 1 (all 3 dropdowns have values), I would like to submit the form and see in my browser's URL field:

(case 1) https://www.example.com/textvalue1/textvalue2/textvalue3

and get the same results (important)!

Similarly, I want the other SEO-friendly URLs to be:

(case 2) https://www.example.com/textvalue1
(case 3) https://www.example.com/textvalue1/textvalue2
(case 4) https://www.example.com/textvalue1/textvalue3

My guess is that I have to do 2 things: First, to convert the ugly URL to a SEO-friendly URL and, second, to treat the new SEO-friendly URL as if it was the old ugly one.

I have tried the following commands in my .htaccess file for case 2 so far but nothing happened:

Options +FollowSymlinks -Indexes -MultiViews
RewriteEngine On
RewriteCond %{THE_REQUEST} \s/results\.php\?sp=([a-zA-Z]+)\s [NC]
RewriteRule ^ /%1? [R=302,L]
RewriteRule ^/([[a-zA-Z]]+)$ /results.php?sp=$1 [L]

Answer 1

As already discussed in the comments to the question I see an issue with the question regarding the rewriting logic. You asked to show a more simplified approach for the situation where all three request arguments are present. Here is a suggestion that also handles the case where only two arguments are specified. You don't even need conditions for that, the matching pattern is enough for such logic:

RewriteEngine on

RewriteCond %{REQUEST} !-f
RewriteCond %{REQUEST} !-d
RewriteRule ^/?([^/]+)/([^/]+)/([^/]+)$ /results.php?sp=$1&ar=$2&in=$3 [END]
RewriteRule ^/?([^/]+)/([^/]+)$ /results.php?sp=$1&ar=$2 [END]

RewriteCond ${QUERY_STRING} ^sp=(.+)&ar=(.+)&in=(.+)$  
RewriteRule ^/results\.php$ /%1/%2/%3 [R=301,END]
RewriteCond ${QUERY_STRING} ^sp=(.+)&ar=(.+)$  
RewriteRule ^/results\.php$ /%1/%2 [R=301,END]

That rule set will work likewise in the http servers host configuration and also in a dynamic configuration file (".htaccess" style file). A general remark here: you should always prefer to place such rules in the http servers host configuration instead of using dynamic configuration files (".htaccess"). Those dynamic configuration files add complexity, are often a cause of unexpected behavior, hard to debug and they really slow down the http server. They are only provided as a last option for situations where you do not have access to the real http servers host configuration (read: really cheap service providers) or for applications insisting on writing their own rules (which is an obvious security nightmare).

In case you experience an "internal server error" (http status 500) using above rule set then chances are that you operate a very old version of the apache http server. In that case replace the END flag with the L flag. You will find a specific hint on the unsupported END flag in your http servers error log file in that case.

Once more: if you can name a logic by which to decide which request argument to assign the second value captured by the second rule if it gets applied, then it would be possible to implement such a condition and handle all three cases you originally mentioned. But without you specifying such logic it is impossible to implement something.