Chainsaw provides an XMLSocketReceiver, which could be used to receive events from log4php, but I don't think they send events in a format that the receiver can process.
I'd suggest logging to a regular text log file and using the latest developer snapshot of Chainsaw, available here: http://people.apache.org/~sdeboy
Use the 'File,Load Chainsaw configuration' option and select 'process a log file'. Then hit the 'open file' to browse to your log file location, and then specify your 'log file format'. A simple one that will always work is:
MESSAGE
That format will just make the entire log line the 'message' field. If you'd like Chainsaw to parse your levels and timestamps etc, that's possible, you just need to use the keywords and match up your format.
Then, click the 'always start Chainsaw with this configuration' check box, and then hit the 'save configuration as' button and save your chainsaw configuration to something like log4php-chainsaw-config.xml.
When you hit ok, you should get a new tab with your log file contents.
Scott