validate_file()

validate_file( string $file, string[] $allowed_files = array() ): int

function validate_file( $file, $allowed_files = array() ) {
	if ( ! is_scalar( $file ) || '' === $file ) {
		return 0;
	}

	// Normalize path for Windows servers.
	$file = wp_normalize_path( $file );
	// Normalize path for $allowed_files as well so it's an apples to apples comparison.
	$allowed_files = array_map( 'wp_normalize_path', $allowed_files );

	// `../` on its own is not allowed:
	if ( '../' === $file ) {
		return 1;
	}

	// More than one occurrence of `../` is not allowed:
	if ( preg_match_all( '#\.\./#', $file, $matches, PREG_SET_ORDER ) && ( count( $matches ) > 1 ) ) {
		return 1;
	}

	// `../` which does not occur at the end of the path is not allowed:
	if ( str_contains( $file, '../' ) && '../' !== mb_substr( $file, -3, 3 ) ) {
		return 1;
	}

	// Files not in the allowed file list are not allowed:
	if ( ! empty( $allowed_files ) && ! in_array( $file, $allowed_files, true ) ) {
		return 3;
	}

	// Absolute Windows drive paths are not allowed:
	if ( ':' === substr( $file, 1, 1 ) ) {
		return 2;
	}

	return 0;
}