Post variables in mysql_query failing

[Mavent]

This works:

$result = mysql_query("SELECT * FROM mydatabase WHERE username = 'billybob'");

 

This does not:

$user = "billybob"

$result = mysql_query("SELECT * FROM mydatabase WHERE username = $user");

 

Shouldn't these be identical?  If I echo $user, I get, of course, "billybob", so does anyone know why the variable isn't working in the query itself?

[Pikachu2000]

String values need to be enclosed in quotes in the query string.

[Mavent]

String values need to be enclosed in quotes in the query string.

 

I could have SWORN that I tried it that way initially, but obviously I didn't, because the second I changed it to '$user' it worked perfectly.  Thanks Pikachu2000!