This converts answer choice numbers to letters.

convertAnswerNumberToLetters.php

function convertAnswerNumberToLetters($number){
		$LETTERS_DICTIONARY = array('a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z');
		$MAX_ITERATIONS = 20;

		$letters = '';

		$numIterations = 0;
		while($number > 0 && $numIterations <= $MAX_ITERATIONS){
			$index = $number;
			for($temp = 0; $temp < $MAX_ITERATIONS && $temp < $number && $temp > 26; $temp -= 26){
				$index = $temp;
			}
			$letters .= $LETTERS_DICTIONARY[$index-1];
			$number -= 26;
			$numIterations++;
		}
		return strtoupper($letters);
}