PHP methods for Implementing a pseudo cache system for files

This question is more about methodology than actual code – lines

I would like to know how to implement a pseudo caching (for lack of a better name) for FILES in php . I have tried to read some articles, but most of them refer to the internal caching system of PHP , and not to what I need which is a FILE cache.

I have several scenarios where I needed such a system applied :

Scenario 1 :

While accessing a post and clicking a link, all the post attachments are collected and added to a zip file for download.

Scenario 2 :

Accessing a post , the script will scan all the content , extract all links, download some matching images for each link (or dynamically prepare one) and then serve those to browser . (but not after checing expiration period ?? )

( Those example uses "post" and "attachment" because i use wordpress and it is wordpress terminology, both currently work for me fine, except they generate the file over and over again. )

My doubts regarding the two scenarios (especially No.2) – How do I prevent the script to do the operation EVERY time the page is accessed ? (in other words , if the file exists , just serve it without looping the whole creating operation again)

My first instinct was call the file with some distinctive (but not load – unique like uniqueid() ) name and then check if it is already on the server , but that presents several problems (like it can already exists as naming , but of another post ..) and also – that should be very resource intensive for a server with 20,000 images .

The second thing I thought was to somehow associate a meta data for those files, but then again, How to implement it ? How to knwo which link is of what image ??

Also, in a case where I check for the file existence on the server , how can I know if the file SHOULD be changed (and therefor recreated ) ?

Since I am refering to wordpress, I thought about storing those images as base64 from binary directly to the DB with the transien_API – but it feels quite clumsy.

To sum up the question . How to generate a file, but also know if it exists and call it directly when needed ?? does my only option is store the file-name in DB and associate it somehow with the post ?? that seems so non efficient ..

EDIT I

I decided to include some example code , as it can help people to understand my dilemma .

function o99_wbss_prepare_with_callback($content,$width='250'){

 $content = preg_replace_callback( '/(http[s]?:[^s]*)/i', 'o99_wbss_prepare_cb', $content );

 return $content;
}

function o99_wbss_prepare_cb($match){
    
    $url = $match[1];
    
    $url = esc_url_raw( $url );//someone said not need ?? 
    
    $url_name = parse_url($url);
        
    $url_name =  $url_name['host'];// get rid of http://..
    $param = '660';
    $url = 'http://somescript/' .  urlencode($url)   . '?w=' . $param ; 
    $uploads = wp_upload_dir();
    //$uniqid = uniqid(); 
    
    $img = $uploads['basedir'] . '/tmp/' . $url_name  .'.jpg' ; // was with $uniqid...
    
   
    if(! @ file_get_contents($url)){
        $url = 'path ' .$url. ' doesn"t exist or unreachable';
        return $url;
        } else {
             $file = file_get_contents( $url );
         }
         // here I will need to make some chck if the file already was generated , and 
        // if so - just serve it ..
         if ( $file) {
         
            file_put_contents( $img, $file ); 
                // Do some other operations on the file and prepare a new one ...
                // this produces a NEW file in the wp-uploads folder with the same name...
              unlink($img);
         }

    return $url;
}