PHP uses resources as a special variable to hold links to external objects, such as files and database connections. Each resource is given an integer id. (Documentation)
Failed Connections
If the database connection fails you'll likely get a "Specified variable is not a valid MySQL-Link resource" error, as Dan Breen mentioned, since the variable that is supposed to hold the resource is null.
$link = mysql_connect('localsoth','baduser','badpass'); // failed connection
$result = mysql_query("SELECT 1", $link); // throws error
Since you're getting a specific resource ID in the error message, the database connection likely closed unexpectedly for some reason. Your program still has a variable with a resource ID, but the external object no longer exists. This may be due to a mysql_close() call somewhere before the call to mysql_query, or an external database error that closed the connection.
$link = mysql_connect();
mysql_close($link);
// $link may still contain a resource identifier, but the external object is gone
mysql_query("SELECT 1", $link);
Reusing Connections
An issue with the mysql extension and mysql_connect() is that by default if you pass the same parameters in successive calls, it will re-use the existing connection rather than create a new one (Documentation). This can be fixed by passing true to the $new_link parameter.
I encountered this myself on a test system where the data from two separate databases in production were combined on to one test server, and in testing the mysql_xxx() function calls walked over each other and broke the system.
$link1 = mysql_connect('localhost','user','pass'); // resource id 1 is given
$link2 = mysql_connect('localhost','user','pass'); // resource id 1 is given again
mysql_close($link2); // the connection at resource id 1 is closed
mysql_query("SELECT 1", $link1); // will fail, since the connection was closed
Using $new_link:
$link1 = mysql_connect('localhost','user','pass'); // resource id 1 is given
$link2 = mysql_connect('localhost','user','pass', true); // resource id 2 is given
mysql_close($link2); // the connection at resource id 2 is closed
mysql_query("SELECT 1", $link1); // the connection at resource id 1 is still open
Edit:
As an aside, I would recommend using the MySQLi extension or PDO instead, if possible. The MySQL extension is getting pretty old, and can't take advantage of any features past MySQL version 4.1.3. Look at http://www.php.net/manual/en/mysqli.overview.php for some details on the differences between the three interfaces.
