Modified from: https://stackoverflow.com/a/2638272/1940255

relative_path.php

<?php

if (!function_exists('relative_path')) {
    function relative_path(string $from, string $to): string {
        // some compatibility fixes for Windows paths
        $from = is_dir($from) ? rtrim($from, '\/') . '/' : $from;
        $to = is_dir($to) ? rtrim($to, '\/') . '/' : $to;
        $from = str_replace('\\', '/', $from);
        $to = str_replace('\\', '/', $to);

        $from = explode('/', $from);
        $to = explode('/', $to);
        
        $relPath = $to;

        foreach ($from as $depth => $dir) {
            // find first non-matching dir
            if ($dir === $to[$depth]) {
                // ignore this directory
                array_shift($relPath);
                continue;
            }

            // get number of remaining dirs to $from
            $remaining = count($from) - $depth;
            
            if ($remaining > 1) {
                // add traversals up to first matching dir
                $padLength = (count($relPath) + $remaining - 1) * -1;
                $relPath = array_pad($relPath, $padLength, '..');
                break;
            }
            
            $relPath[0] = './' . $relPath[0];
        }

        return implode('/', $relPath);
    }
}