<input type='text' name='en_ch_date' value='<? echo date(("Ymd"), strtotime("-3 days")); ?>'>

This correctly outputs 20080204 with today being 20080207.

I want to put a variable in for -3 which will change but currently has a value of 100. How do I go about putting a variable in this function?

Also how would you add the date() function in a sql query like the one below?

 VALUES ('$_POST[mach_id]','Engine Oil','date("Ymd")','$_POST[en_ch_hours]')";

Look directly for the database time functions.
For mysql:

Here is an example that uses date functions. The following query
selects all rows with a date_col value from within the last 30 days:    

   mysql> SELECT something FROM tbl_name
    -> WHERE DATE_SUB(CURDATE(),INTERVAL 30 DAY) <= date_col;

http://dev.mysql.com/doc/refman/5.0/...functions.html

Also how would you add the date() function in a sql query like the one below?

VALUES ('$_POST[mach_id]','Engine Oil',date('.date("Ymd").'),'$_POST[en_ch_hours]')";

Thank you for your help.

In the first situation, I just want to replace the -3 with a variable like:

<input type='text' name='en_ch_date' value='<? echo date(("Ymd"), strtotime("$number days")); ?>'>
How do I do this?

exactly the way you just did, and by defining

PHP Code:

$number="-3"; 


before outputting this HTML