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-