Source

File: wp-admin/includes/file.php

function list_files( $folder = '', $levels = 100, $exclusions = array() ) {
	if ( empty( $folder ) ) {
		return false;
	}

	$folder = trailingslashit( $folder );

	if ( ! $levels ) {
		return false;
	}

	$files = array();

	$dir = @opendir( $folder );
	if ( $dir ) {
		while ( ( $file = readdir( $dir ) ) !== false ) {
			// Skip current and parent folder links.
			if ( in_array( $file, array( '.', '..' ), true ) ) {
				continue;
			}

			// Skip hidden and excluded files.
			if ( '.' === $file[0] || in_array( $file, $exclusions, true ) ) {
				continue;
			}

			if ( is_dir( $folder . $file ) ) {
				$files2 = list_files( $folder . $file, $levels - 1 );
				if ( $files2 ) {
					$files = array_merge( $files, $files2 );
				} else {
					$files[] = $folder . $file . '/';
				}
			} else {
				$files[] = $folder . $file;
			}
		}

		closedir( $dir );
	}

	return $files;
}