function str_startswith ( $candidate, $search_str )
{
    return substr( $candidate, 0, strlen( $search_str) ) == $search_str;
}
The software uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the software does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.