<?php

$str = "abcdef";
$len = strlen($str);

$arr = array();
for($count = 0; $count < ($len - 1); $count++)
{
    $arr[] = $str[$count].$str[$count+1];
}

print_r($arr);

?>

Array
(
    [0] => ab
    [1] => bc
    [2] => cd
    [3] => de
    [4] => ef
)