<?php
function encriptar($var)
{
	$search  = array('E', 'D', 'C', 'B', 'A'); 
	$replace = array('A1B', 'E', 'D', 'C', 'B');
	
	$pos = array_search($var, $search);
	if ($pos !== FALSE)
		$b = str_replace($search[$pos], $replace[$pos], $var);
	
	echo $b;
}

encriptar('E');