Connecting the World, Two Servers at a Time - MySQL

Everyone who’s reading this article has probably used PHP to connect to a single database, return data to a pointer variable and display it in an HTML wrapper. You’ve probably also used PHP to take data from an HTML form and insert that data into a table using SQL insert statements. What you likely have not done is performed both of those operations on separate databases nearly simultaneously in the same script.

To perform this trick, we’re going to need to make use of the database connection identifier, something which is likely ignored in simpler pages. One very convenient feature of PHP is the assumption that if you’ve only opened up one database connection, you’re going to use it to perform any further database operations and you don’t need to specify it in database functions. However, that connection identifier is available if you need perform operations on two databases at once.

To use the connection identifier, simply set the result of your mysql_connect() function to a variable:

$source_cnx = mysql_connect("source_db", "uname", "pass");

You can then use this connection identifier in any subsequent database related calls such as mysql_select_db() and mysql_query(). It’s important to remember to call mysql_close() for every connection that’s opened, as well. (This isn’t a complete list of functions that can take the connection identifier argument, but they are the most commonly used. For a complete list, please see the PHP manual at www.php.net.) Because the connection identifier is an optional argument in these functions, it is typically the last argument in the function call.

After setting up your first connection, you would set up another in the same way, using a different connection identifier variable:

$target_cnx = mysql_connect("target_db", "uname", "pass");

Using this information we can set up the shell of connections for a multiple database connection script with some error checking as follows:

if (!($srcCnx = mysql_connect($srcHost, $srcUname, $srcPass)))
echo "Unable to connect to $srcHost.<BR>";
if (!mysql_select_db($srcDB, $srcCnx))
echo "Unable to open database $srcDB on $srcHost.<BR>";
if (!($tgtCnx = mysql_connect($tgtHost, $tgtUname, $tgtPass)))
echo "Unable to connect to $tgtHost.<BR>";
if (!mysql_select_db($tgtDB, $tgtCnx))
echo "Unable to open database $tgtDB on $tgtHost.<BR>";
. . . (perform SQL transactions on databases) . . . 
mysql_close($srcCnx);
mysql_close($tgtCnx);

>>> More MySQL Articles          >>> More By Stephen Junker

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