Gareth Heyes has another post to his site on the topic of "non-alpha PHP code", this time getting a bit more into the process and how his examples are parsed by PHP into more familiar functionality.

My first post on PHP non-alpha numeric code was a bit brief, in the excitement of the discovery I failed to detail in depth the process. I’ve decided to follow up with a tutorial and hopefully explain the process better for anyone wanting to learn or improve the technique. The basis of PHP non-alphanumeric code is to take advantage of the fact that PHP automatically converts Arrays into a string “Array” when using in a string context.

He includes some basic examples showing how, with just a combination of things like "+", "_" and "[" or "]" you can reproduce similar output to echoing out an array and use that "Array" output string to get to other strings (like the letter "B"). There's also a more lengthy example showing how to build up the string "print 1+1" and have it execute using this technique.