PHP how to use $_Session to transfer a value stored in a username text box to a sql query

I have an Android app that allows the user to look up books.

The app has login page that queries a PHP script (hosted in wamp) that checks the username and password and if it exists takes you to the main page.
The main page runs a PHP script that contains a SQL script that returns a list users of the app in a listview . Upto now this works ok.

However what i want to do is just return all users but not the user that logged in. I have read that $_Session can be used as a global variable. So I could use $_Session to store the username and then run the query to get all users <> user thats logged in.
Can anyone give me an example or point me to where i can find out how to use $_Session variable for this use?

For example im on the right track using it like this?

FROM users
WHERE EXISTS (SELECT 0 FROM available A1, available A2
WHERE users.uid <>$_Session [username])") or die(mysql_error())
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Hopefully, your table has a primary key.  What you need to do is, upon login, retrieve the user's Primary key.  Then use that primary key to select all the other users, fitering out the current user.

So, if you table was
`firstname` varchar(50),
`lastname` varchar(50),
`username` varchar(50),
`password` varchar(50),

Upon login, save the id onto a session variable:
// you must call this before tampering with $_SESSION
# Then your query will basically be
# SELECT id FROM users WHERE username='...' and password='...'
#if successful, you will have only one record.  Retrieve that record and save it onto $_SESSION['user_id'].

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Then on your other page, (which is your subsequent query) you would need:

# connect to db

#craft your sql that will exclude the user that is currently logged in.
$sql = sprintf("select id, username FROM users WHERE id <> %d", $_SESSION['user_id']);

# now execute the $sql query above

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Ray PaseurCommented:
If you're new to PHP and want some good learning resources, this article can help.

$_SESSION (case-sensitive) is a PHP superglobal variable.  Used properly it can provide "stateful" information that is carried by the server between requests.  To fully understand how it works, you must understand the nature of HTTP Client/Server protocols.

Once you've "got" that, the PHP session is very easy to use correctly!

Session data can be used in SQL queries, but you may find yourself dealing with fiddly punctuation because of all the quote marks, and there are fairly serious security implications of using external data in a query string.  Recommend you start with the first article and build up your experience in PHP a little at a time.  Or if time does not permit, consider hiring a professional developer to help.
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