根据PHP中输入的字符操作单个字符串

I'm wondering how to manipulate individual characters in a string in PHP. I've searched all over and can't find anything that answers this type of question. I want to take a string that's submitted using an input form that, for example, says "this is a sentence" and then create an output that takes the first character of every word and puts it at the end of that word: "hist si a entences". I'm assuming the first step would to be to take the string and turn it into an array using explode(), but I'm really confused as to how to perform the actual manipulation? Any help here would be great! Thank you!I'm wondering how to manipulate individual char