Get Homebrew installed on your mac if you don't already have it
Install highlight. "brew install highlight". (This brings down Lua and Boost as well)
Highlight your source. "highlight -O rtf myfile.php | pbcopy". This will highlight the source code in RTF format and copy the result of that operation to your Mac's clipboard.
Paste into your slide in Keynote. Keynote will create a new text box with your highlighted code. You can adjust the width as needed, and adjust the font size with cmd+ and cmd-
If you want to use Alfred to format code from the clipboard (based on @mazuhl's comment above): https://dl.dropboxusercontent.com/u/1806977/Highlight%20Code.alfredworkflow
In case it doesn't work: Make sure $PATH is set accordingly. Alfred apparently does not know about your $PATH, that's why the workflow starts with
export PATH=$PATH:/usr/local/bin
. However, ifhighlight
is on a different path, this has to be changed.