Hmm... There's no need to complicate things. The basics work great always.
$string = 'abcdef';
$len = strlen( $string );
$x = 0;
Forward Direction:
while ( $len > $x ) echo $string[ $x++ ];
Outputs: abcdef
Reverse Direction:
while ( $len ) echo $string[ --$len ];
Outputs: fedcba