Source

File: wp-includes/functions.php

function recurse_dirsize( $directory, $exclude = null, $max_execution_time = null ) {
	$size = 0;

	$directory = untrailingslashit( $directory );

	if ( ! file_exists( $directory ) || ! is_dir( $directory ) || ! is_readable( $directory ) ) {
		return false;
	}

	if (
		( is_string( $exclude ) && $directory === $exclude ) ||
		( is_array( $exclude ) && in_array( $directory, $exclude, true ) )
	) {
		return false;
	}

	if ( null === $max_execution_time ) {
		// Keep the previous behavior but attempt to prevent fatal errors from timeout if possible.
		if ( function_exists( 'ini_get' ) ) {
			$max_execution_time = ini_get( 'max_execution_time' );
		} else {
			// Disable...
			$max_execution_time = 0;
		}

		// Leave 1 second "buffer" for other operations if $max_execution_time has reasonable value.
		if ( $max_execution_time > 10 ) {
			$max_execution_time -= 1;
		}
	}

	$handle = opendir( $directory );
	if ( $handle ) {
		while ( ( $file = readdir( $handle ) ) !== false ) {
			$path = $directory . '/' . $file;
			if ( '.' !== $file && '..' !== $file ) {
				if ( is_file( $path ) ) {
					$size += filesize( $path );
				} elseif ( is_dir( $path ) ) {
					$handlesize = recurse_dirsize( $path, $exclude, $max_execution_time );
					if ( $handlesize > 0 ) {
						$size += $handlesize;
					}
				}

				if ( $max_execution_time > 0 && microtime( true ) - WP_START_TIMESTAMP > $max_execution_time ) {
					// Time exceeded. Give up instead of risking a fatal timeout.
					$size = null;
					break;
				}
			}
		}
		closedir( $handle );
	}
	return $size;
}