Few string questions..

[|2eM!x]

First, how can i replace part of a string, in a string? str_replace(just a guess)?

Secondly, how can i determine what the letter is of a character anywhere in a string?
ex:
yaytaweuih

and then i want to find the 5th letter which is a

[{yak}]

Yeah, str_replace() will work, theirs some other alternatives like preg, but the manual can probably explain it better than I, and has some alternative links as well.
str_replace() (http://us3.php.net/str_replace)
For your second question, sounds like substr() is what you're looking for. substr() (http://us3.php.net/manual/en/function.substr.php)

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You can also use preg_replace() (http://www.php.net/preg_replace) and preg_replace_callback() (http://www.php.net/preg_replace_callback) to find and replace strings matching regular expressions.