simple string changer

simple string changer
i want to change string 1234567890 into abcdefghij

   a$=1234567890;
  $code = str_replace('1', "a", $a);
  $code = str_replace('2', "b", $a);
  $code = str_replace('3', "c", $a);
  $code = str_replace('4', "d", $a);
  $code = str_replace('5', "e", $a);
  $code = str_replace('6', "f", $a);
  $code = str_replace('7', "g", $a);
  $code = str_replace('8', "h", $a);
  $code = str_replace('9', "i", $a);
  $code = str_replace('0', "j", $a);

echo $a;

PHP:

but it is not work

it showing 1234567890

 

<?php

$a = 1234567890;

$numbers = array("1", "2", "3", "4", "5", "6", "7", "8", "9", "0");
$letters = array("a", "b", "c", "d", "e", "f", "g", "h", "i", "j");

$change = str_replace($numbers, $letters, $a); 

echo $change; 

?>

PHP:

 

Thanks for that ... it is what that i want