how to reverse engineer some simple obfuscation

cwp.php

<?php
//grabbed the code from phpQUERY-oneFile.php which houses a good chunk of the setup scripts that run cwp
// found this method of obfuscation - just getting the ord (ascii value) for a character and then shifting values.
/* rewrote their script to make logical. The function decryptme must be converted back to the original function name as its called further down the line in the obnfuscated code. */
/* so far havent found anything objectionable in their code - just looking for obvious backdoors */
if(!function_exists("decryptme")) 
{ function decryptme($obfuscated) { 
	$initialize=""; 
	$counter=0; 
	$length=strlen($obfuscated); 
	while($counter < $length) { 
		if($obfuscated[$counter] == ' ') { 
			$initialize.=" "; } 
		else if($obfuscated[$counter] == '!') { 
			$initialize.=chr((ord($obfuscated[$counter+1])-ord('A'))*16+(ord($obfuscated[$counter+2])-ord('a')));
			$counter+=2;} 
		else { $initialize.=chr(ord($obfuscated[$counter])+1); } 
		$counter++; 
		} 
	return $initialize; 
	} 
}
echo(decryptme('du`k!Ci`..............'));


/* original code */

if(!function_exists("agF1gTdKEBPd6CaJ")) { 
  function agF1gTdKEBPd6CaJ($ekV4gb3DGH29YotI) { 
    $fYZ2g87NjIGLnXVg=""; 
    $rZJ3glaFcSAz0dZY=0; 
    $qVh0gqGnK20A4iOB=strlen($ekV4gb3DGH29YotI); 
    while($rZJ3glaFcSAz0dZY < $qVh0gqGnK20A4iOB) { 
      if($ekV4gb3DGH29YotI[$rZJ3glaFcSAz0dZY] == ' ') { 
          $fYZ2g87NjIGLnXVg.=" "; } 
        else if($ekV4gb3DGH29YotI[$rZJ3glaFcSAz0dZY] == '!') { 
          $fYZ2g87NjIGLnXVg.=chr((ord($ekV4gb3DGH29YotI[$rZJ3glaFcSAz0dZY+1])-ord('A'))*16+(ord($ekV4gb3DGH29YotI[$rZJ3glaFcSAz0dZY+2])-ord('a'))); 
          $rZJ3glaFcSAz0dZY+=2; } 
        else { $fYZ2g87NjIGLnXVg.=chr(ord($ekV4gb3DGH29YotI[$rZJ3glaFcSAz0dZY])+1); } 
      $rZJ3glaFcSAz0dZY++; 
    } 
    return $fYZ2g87NjIGLnXVg; 
    
  } }eval(agF1gTdKEBPd6CaJ('du`k!Ci`