Warning: strpos() expects parameter 1

/**
 * File validates against allowed set of defined rules.
 *
 * A return value of '1' means that the $file contains either '..' or './'. A
 * return value of '2' means that the $file contains ':' after the first
 * character. A return value of '3' means that the file is not in the allowed
 * files list.
 *
 * @since 1.2.0
 *
 * @param string $file File path.
 * @param array $allowed_files List of allowed files.
 * @return int 0 means nothing is wrong, greater than 0 means something was wrong.
 */
function validate_file( $file, $allowed_files = '' ) {
    if ( false !== strpos( $file, '..' ))
        return 1;


    if ( false !== strpos( $file, './' ))
        return 1;


    if (!empty ( $allowed_files ) && (!in_array( $file, $allowed_files ) ) )
        return 3;


    if (':' == substr( $file, 1, 1 ))
        return 2;


    return 0;
}