// Created on savesnippets.com · https://savesnippets.com/ZefuA8KkWogArA
<?php
function safeJoin(string $base, string ...$parts): string {
    $base = rtrim(realpath($base) ?: $base, DIRECTORY_SEPARATOR);
    $joined = $base . DIRECTORY_SEPARATOR . implode(DIRECTORY_SEPARATOR, $parts);
    // Resolve "." and ".." segments without requiring the file to exist.
    $segments = [];
    foreach (explode(DIRECTORY_SEPARATOR, $joined) as $s) {
        if ($s === '' || $s === '.') continue;
        if ($s === '..') { array_pop($segments); continue; }
        $segments[] = $s;
    }
    $resolved = DIRECTORY_SEPARATOR . implode(DIRECTORY_SEPARATOR, $segments);
    if (!str_starts_with($resolved, $base . DIRECTORY_SEPARATOR) && $resolved !== $base) {
        throw new RuntimeException('Path traversal attempt blocked');
    }
    return $resolved;
}

echo safeJoin('/var/www/uploads', 'user123', 'avatar.jpg');
// /var/www/uploads/user123/avatar.jpg
// safeJoin('/var/www/uploads', '../../etc/passwd')   →  throws