php preg_replace array but only in the initial string

Question

This scripts

<?php
$pat = array ( '/A/', '/B/');
$rep = array ( 'B', 'C');
print preg_replace($pat, $rep, 'AAB');
?>

I would liket to print 'BBC' ('B' replaces 'A' and 'C' replaces only the initials 'B')

But it prints 'CCC' ('B' replaces 'A' and 'C' replaces 'B' and the 'A' previously replaced by 'B')

If I tried something like this script

<?php
$pat = array ( '/A/', '/B/');
$rep = array ( 'B', 'C');
print preg_replace($pat, $rep, 'AAB', 1);
?>

But it prints 'CAB'...

Thanks.

Answer 1

This should work :) You just change it's order, so it replaces B before A, and that's it

<?php
$pat = array ( '/B/', '/A/');
$rep = array ( 'C', 'B');
print preg_replace($pat, $rep, 'AAB');
?>