Say I have an image path 'image_path/123.jpg' stored in my database and I need that image's thumbnail, which is in 'image_path/200x200/123.jpg'. Just pass the path and subfolder name to this helper function and it will insert the subfolder in the string.

get_thumbnail.php

<?php

function get_thumbnail($img_path, $folder = ''){
    
    // Should we just return the given $img_path?
    if(empty($folder) || !strstr($img_path, '/')){
        return $img_path;
    }
    
    // Insert folder name right before image filename
    $img_path = explode('/', $img_path);
    $file_name = array_pop($img_path);
    array_push($img_path, $folder);
    array_push($img_path, $file_name);
    return implode('/', $img_path);
}

// Try it out
echo get_thumbnail('/image_path/thumbs/12312.jpg', '200x200') . PHP_EOL;
echo get_thumbnail('/image_path/thumbs/12312.jpg') . PHP_EOL;
echo get_thumbnail('/som/other/path/test.jpg', 'folder/300') . PHP_EOL;