MySQL is not inserting a backslash

You need to escape your backslash :

INSERT INTO gender
(sex, date) VALUES (
'male are allowed \\ female are not allowed',
"2012-10-06")

Reference (with the list of all characters you must escape for mysql)

Handle backslashes and all control characters in mysql load data infile tool:

Step 1, create your table:

mysql> create table penguin (id int primary key, chucknorris VARCHAR(4000));
Query OK, 0 rows affected (0.01 sec)

Step 2, create your file to import and put this data in there.

1   spacealiens are on route
2   scramble the nimitz\
3   \its species 8472
4   \\\\\\\\\\\\\\\\\\
5   Bonus characters:!@#$%^&*()_+=-[]\|}{;'":/.?>,< anything but tab

Step 3, insert into your table:

mysql> load data local infile '/home/el/foo/textfile.txt' into table penguin 
       fields terminated by '\t' lines terminated by '\n' 
       (@col1, @col2) set id=@col1, chucknorris=@col2;
Query OK, 4 rows affected, 1 warning (0.00 sec)
Records: 4  Deleted: 0  Skipped: 0  Warnings: 1

Step 4, and of course, it causes this strange output:

mysql> select * from penguin;
+----+-----------------------------------------------------------------+
| id | chucknorris                                                     |
+----+-----------------------------------------------------------------+
|  1 | spacealiens are on route                                        |
|  2 | scramble the nimitz                                             |
|  3 |                                                                 |
|  4 | \\\\\\\\\                                                       |
|  5 | Bonus characters:!@#$%^&*()_+=-[]|}{;'":/.?>,< anything but tab |
+----+-----------------------------------------------------------------+

Step 5, analyze the warning:

mysql> show warnings;
+---------+------+--------------------------------------------------------+
| Level   | Code | Message                                                |
+---------+------+------------------------------------- ------------------+
| Warning | 1262 | Row 2 was truncated; it contained more data than there |
|         |      | were input columns                                     |
+---------+------+--------------------------------------------------------+
1 row in set (0.00 sec)

Step 6, think about exactly what went wrong:

The backslash to the left of nimitz caused the mysql load data parser to concatenate the end of line 2 with the beginning of line 3. Then it bumped up against a tab and put 'scramble the nimitz\n3 into row 2.

The rest of row 3 is skipped because the extra words its species 8472 do not fit anywhere, it produces the warning you see above.

Row 4 had 18 backslashes, so there is no problem, and shows up as 9 backslahes because each was escaped. Had there been an odd number, the error on row 2 would have happened to row 4.

The bonus characters on row 5 came through normally. Everything is allowed except tab.

Step 7, reset table penguin:

mysql> delete from penguin;

Step 8, load into your table with the fields escaped by clause:

mysql> load data local infile '/home/el/foo/textfile.txt' into table penguin 
       fields terminated by '\t' escaped by '\b' 
       lines terminated by '\n' (@col1, @col2) set id=@col1, 
       chucknorris=@col2;

Query OK, 5 rows affected (0.00 sec)
Records: 5  Deleted: 0  Skipped: 0  Warnings: 0

Step 9, select from your table, interpret the results:

mysql> select * from penguin;
+----+------------------------------------------------------------------+
| id | chucknorris                                                      |
+----+------------------------------------------------------------------+
|  1 | spacealiens are on route                                         |
|  2 | scramble the nimitz\                                             |
|  3 | \its species 8472                                                |
|  4 | \\\\\\\\\\\\\\\\\\                                               |
|  5 | Bonus characters:!@#$%^&*()_+=-[]\|}{;'":/.?>,< anything but tab |
+----+------------------------------------------------------------------+
5 rows in set (0.00 sec)

And now everything is as we expect. The backslash at the end of line 2 does not escape the newline. The backslash before i on row 3 doesn't do anything. The 18 backslashes on row 4 are not escaped. And the bonus characters come through ok.

you can use this code :

$yourVariable = addcslashes($_POST["your param"],"\\");

for example in my web form i want insert local directory :

$localAddress = addcslashes($_POST["localAddress"],"\\");