Write multiple backslashes

Hi,

I have a file in which I need to interchange two regular expressions every hour
depending on what data will be processed. In other words, I will replace the one
regular expression with the other (they are never together in the same file).
This file is a configuration file for attatched hardware (not written by myself
but by the hardware manufacturer).

Below is what it should look like in the file:
IGNORE_TMP=^()$
or
IGNORE_TMP=^(\\*.*\\*)$" (Yes, this one has 2 backslashes) 

Now, the first regular expression I can write to the file like this with no
problems:
$first_regexp = "/IGNORE_TMP=\\^\\(\\)\\$/" ;
which will give me:
IGNORE_TMP=^()$.

However, I cannot seem to get it right to write the second regular expression
correctly to get 2 backlashes. I can get 1 with no problem:
$second_regexp = "IGNORE_TMP=^(\\*.*\\*)$" ;

### Here is just the rest of the code for reference ###
$block_file = file_get_contents ( $tmp_file ) ;
$block_regexp = preg_replace ( $old_block, $new_block, $block_file ) ;
file_put_contents ( $tmp_file, $block_regexp ) ;
$remove_blank_lines = exec ( 'sed -i "/^[[:space:]]*$/d" /etc/scripts/motor_control/temperature.conf' ) ;
#######################################################

Any pointers?

Thank you

Danny