php with single and double quotation marks

Hey all

One thing that I've been meaning to look at for a while, and now am forced to look at, is apostrophies and them in php.

For example:

If someone enters into a text field "My Dad's brand new car". The value of that text field goes to a database..which the code would obviously look something like this

PHP:

$subject = $_POST["subject"];
$query = "INSERT INTO myDBtable (subject) VALUES ('$subject')";

But of course, thats gonna cause a problem because what mysql is seeing is

INSERT INTO myDBtable (subject) VALUES ('My Dad's brand new car')

the apostrophie in "Dad's" is causing it to think thats its finished.

I've seen people using slashes (/) but it hasn't been explained properly.
Even if someone has a reference to another website, would be helpful.