$cell = mb_substr(strip_tags($cell),0,30,"UTF-8");

http://blog.xuite.net/chenbruse/bruse/13351768-%E8%A7%A3%E6%B1%BAPHP+substr%E5%88%87%E5%89%B2%E4%B8%AD%E6%96%87%E5%AD%97%E5%95%8F%E9%A1%8C