<?php
$numarr=array('A' => 0,
'B' => 1,
'C' => 2,
'D' => 3,
'E' => 4,
'F' => 5,
'G' => 6,
'H' => 7,
'I' => 8,
'J' => 9);
$narr = array_flip($numarr);

$num = 100;
$arr = str_split($num);

$str = '';
foreach($arr as $s)
   $str .= $narr[$s];
echo $str . "\n"; //BAA
?>