I used the following charset: $_=+(); As you can see only 7 chars. In the end the function log will be created,because this won't exceed the memory limit. In the beginning I created the php function assert and it was 99 mb large php file, way too big to get executed.

He uses a few tricks to get the language to create strings (counting to infinity), changing the letters in the resulting string, He includes the steps in the process and a large block of the non-alpha PHP code at the end of the post, a 46KByte block that echoes "log(12)".