<?php
function is_arithmetic($arr)
  {
   $delta = $arr[1] - $arr[0];
   for($index=0; $index<sizeof($arr)-1; $index++)
    {
        if (($arr[$index + 1] - $arr[$index]) != $delta)
        {
             
             return "Not an arithmetic sequence";
        }       
    }
    return "A arithmetic sequence";
}
$my_arr1 = array(6, 7, 9, 11);
$my_arr2 = array(5, 7, 9, 11);

print_r(is_arithmetic($my_arr1)."\n");
print_r(is_arithmetic($my_arr2)."\n");
?>