Ever wondered what your PHP application and MySQL actually do? An experimental mysqlnd branch will give you full access to the network communication stream. Using a custom PHP stream filter you can then intercept the communication ... but let's start at the beginning:

When talking about mysqlnd - the mysql native driver for PHP - we always mention the fact it's native in a way that we're, when possible, using PHP infrastructure. The most common example here is the memory management. By directly using PHP's memory we can avoid unnecessary copies of data from the MySQL Client Library's memory into PHP memory.

<?php
$mysqli = mysqli_connect("localhost", "root", "", "test");
$stream = mysqli_conn_to_stream($mysqli);
stream_filter_append($stream, "mysql.server", STREAM_FILTER_READ);
?>

But there's more what we're doing. We're also using PHP's stream abstraction layer. From a development perspective the benefit is that we're using a tested abstraction from different stream implementations by different operating systems instead of writing our own. But, again, there's more to it: We can export the communication stream to PHP userland. We hesitated about exporting it for some time as it can be quite dangerous and you might easily corrupt the client-server- communication.

As Ulf mentioned during his IPC talk I recently pushed a mysqlnd branch to launchpad which adds a userspace function to mysqli which returns a PHP stream for a connection. Using that stream you can now send your own requests to the server and wait for the response. That might be nice in a way, but I guess you most likely won't have use for that. PHP streams allow you to do more: PHP streams give you the possibility to add filters to a stream. These filters allow you to intercept packages which are sent or received , read them, change them or do whatever you like. A very simple filter can be found on the launchpad site, mentioned above. That filter simply prints the information after replacing unprintable (binary) characters by dots.

Once again: Just a small step, the next one is to decode the MySQL protocol. For that I've written a simple decoder for the MySQL protocol, not complete, but enough to give an idea. The script, including the decoder and some sample code using it, is, as a sample, part of the branch. When running you will get some output like

Query:
-> 0 59: QUERY: SELECT TABLE_SCHEMA FROM INFORMATION_SCHEMA.TABLES LIMIT 1
<- 1 1: DATA
<- 2 52: FIELD INFO
<- 3 5: EOF
<- 4 19: DATA
<- 5 5: EOF
Invalid Query:
-> 0 29: QUERY: ghdfjtgfdrs tztr ttgdszthtdr
<- 1 183: ERROR: 2000You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'ghdfjtgfdrs tztr ttgdszthtdr' at line 1
Prepare:
-> 0 61: PREPARE: SELECT TABLE_NAME FROM INFORMATION_SCHEMA.STATISTICS LIMIT 2
<- 1 12: OK
<- 2 52: FIELD INFO
<- 3 5: EOF
Ping:
-> 0 1: PING
<- 1 7: OK
Execute:
-> 0 11: EXECUTE
<- 1 1: DATA
<- 2 52: FIELD INFO
<- 3 5: EOF
<- 4 15: OK
<- 5 15: OK
<- 6 5: EOF
EOF
-> 0 5: CLOSE_STMT
-> 0 1: QUIT

As one can see: The protocol isn't fully decoded yet so this all might be extended but for me it served the purpose well enough. For making real use out of this we're thinking about exporting the protocol decoder which exists within mysqlnd to PHP userland.

What are your ideas for such a feature? - Sending different queries to different servers? Rewriting queries? Sharding? Replication? Easy scaling of your application while refactoring your application? Let us know!