Source

File: wp-admin/includes/misc.php

function save_mod_rewrite_rules() {
	if ( is_multisite() ) {
		return;
	}

	global $wp_rewrite;

	// Ensure get_home_path() is declared.
	require_once ABSPATH . 'wp-admin/includes/file.php';

	$home_path     = get_home_path();
	$htaccess_file = $home_path . '.htaccess';

	/*
	 * If the file doesn't already exist check for write access to the directory
	 * and whether we have some rules. Else check for write access to the file.
	 */
	if ( ( ! file_exists( $htaccess_file ) && is_writable( $home_path ) && $wp_rewrite->using_mod_rewrite_permalinks() ) || is_writable( $htaccess_file ) ) {
		if ( got_mod_rewrite() ) {
			$rules = explode( "\n", $wp_rewrite->mod_rewrite_rules() );
			return insert_with_markers( $htaccess_file, 'WordPress', $rules );
		}
	}

	return false;
}