Learning how to use functions in PHP should not be like an American-style spelling bee where you need to memorise wholesale large chunks of information and be able to reel it off as and when it is required. Instead, you will find it is better to keep a general function overview in the back of your head so that you know roughly what functions are available, then refer back here for more specific usage information as necessary.
Unless you really like a challenge, don't bother trying to remember parts of the ASCII table. Not only is it not very helpful with the chr() and ord() functions close to hand, but it's also available online at www.asciitable.com.
If this was helpful, please take a moment to tell others about Hacking with PHP by tweeting about it!