you only have to escape quotes if you intend for them to be used/output... in this case you dont
$query = "SELECT user_info.username, user_info.password, user_info.UserID, user_info.EmailAddress, user_info.Address, user_info.Postcode, user_info.FirstName, user_info.LastName FROM user_info WHERE user_info.username = $username";
theres also really no reason to prefix your field selections with "user_info" since you are only querying one table but thats really a matter of preference