<?php
function letter_range($length)
{
    $data_range = array();
    $letters = range('A', 'Z');
    for($i=0; $i<$length; $i++)
    {
        $position = $i*26;
        foreach($letters as $ii => $letter)
        {
            $position++;
            if($position <= $length)
                $data_range[] = ($position > 26 ? $data_range[$i-1] : '').$letter;
        }
    }
    return $data_range;
}
print_r(letter_range(7));
?>