dude, it's like this:
'' - anything in single quotes is taken as a literal value.
"" - anything in double quotes is evaluated -- that is, PHP decides whether the value is a string or integer type depending on the context it's used in.
quotes should always be escaped when using them in db. ie. \" and \'
use the function 'addslashes()' to return a string with the proper slashes [escaped characters].
hope this helps. there's a good explanation in the php manual.
php.net