finding if $key has value

here is what i try to accpmplish: $key "name's" in an array are: a1, a2, a3, .... . $value dependes weather it has been checked, selected or entered. while (list($k, $v) = each($data)) {

    for ($i=0; $i<count($k); $i++) {
      if (substr ($k[$i], 0, 1) == 'a') {
        if ($k == !$v) { // <-- ????
          $sectionA_error = 1;
          } else {
            $sectionA_error = 0;
          }
       }
    }		

} if ($sectionA_error) {

     echo "Nothing entered.\n<br>";
        } else {
            echo "at least one entry.\n<br>";
        }

what i'm trying to get is: if there is a least one or doesn't matter how many values error = 0, if there is no value than error =1. cheers, Peter