语句在mysql查询没问题，可以显示查询结果的.
$sql = mysql_query('select attach.* , category.* from attach, category where attach.id = substring_index(substring_index(substr(category.ext,locate('attach',category.ext)),'',4),'',-1) order by rand() limit 5');
如果去掉
substring_index(substring_index(substr(category.ext,locate('attach',category.ext)),'',4),'',-1)
就没问题，怎么解决呢？
回复讨论(解决方案) 你在被单引号括起的串中使用了未转义的单引号，难道不报错吗？
$sqlstr = select attach.* , category.* from attach, category where attach.id = substring_index(substring_index(substr(category.ext,locate('attach',category.ext)),'',4),'',-1) order by rand() limit 5
sql;
$sql = mysql_query($sqlstr);
谢谢啊