PHP in a Tweet is a code golf game like 140byt.es, where you should write a valid PHP code in 140 bytes (the size of a tweet) and post it on Twitter.
To add your tweet here fork the website on GitHub and add your tweet to the _posts directory following current examples.
echo "#php"; or return "#error"; but still keeping your tweet a well-formed PHP code<? and <?php) and close (?>) tags are not recommended