Simple numerical encryption/decryption

<?php
function num2abc($chars = "") 
{ 
	$str = '';
	$chars = explode(",",$chars);
	$alpha = range('a','z'); 	
	foreach($chars as $char) 
	{ 
		$str .= ($char==-1) ? ' ' : $alpha[$char]; 
	} 
	return $str; 
}

function abc2num($chars = "")
{
	$arr = array();
	$numbers = array_combine(range('a','z'),range(0,25));
	$letters = str_split($chars);
	foreach ($letters as $letter)
	{
		$str .= ($letter==' ') ? ',-1' : ',' . $numbers[$letter];
	}
	return trim($str,',');
}

/*
echo num2abc('0,1,2,-1,23,24,25'); // abc xyz
echo abc2num('abc xyz'); // 0,1,2,-1,23,24,25
*/
?>