Why is the SQL query code blank in php?

Why is the SQL query code blank in php? Why is this SQL query statement blank when it is put into php? The statement is no problem in mysql Query, and the query result can be displayed. $ SQL & nbsp; = & nbsp; mysql_query ('select & nbsp; attach. * & nbsp;, & nbsp; category: Why is the SQL query code blank in php?
Why is this SQL query statement blank when it is put into php?
The statement is no problem in mysql Query, and the query result can be displayed.

$ SQL = mysql_query ('select attach. *, category. * from attach, category where attach. id = substring_index (substr (category. ext, locate ('"attach"', category. ext), '"', 4), '"',-1) order by rand () limit 5 ');

If you remove
Substring_index (substr (category. ext, locate ('"attach"', category. ext), '"', 4), '"',-1)
No problem. how can this problem be solved?

------ Solution --------------------
You have used unescaped single quotes in the string enclosed by single quotes. isn't it an error?

$ Sqlstr = <SQL
Select attach. *, category. * from attach, category where attach. id = substring_index (substr (category. ext, locate ('"attach"', category. ext), '"', 4), '"',-1) order by rand () limit 5
SQL;
$ SQL = mysql_query ($ sqlstr );