I would use names for my form like this:
<input type="text" name="formitem[0]['lastname']">
<input type="text" name="formitem[0]['address']">
...
<input type="text" name="formitem[1]['lastname']">
<input type="text" name="formitem[1]['address']">
...
<input type="text" name="formitem[20]['lastname']">
<input type="text" name="formitem[20]['address']">
And process it with a simple foreach:
<?php
$formitem = $_POST['formitem];
foreach ($formitem as $recno=>$v){
// scrub your data here somewhere. I'm writing this without any, and assuming magic_quotes_gpc is turned on.
$query = "insert into sometable (";
$query.= implode(",",array_keys($v));
$query.= ") values ("'";
$query.= implode("','",array_values($v));
$query.="')";
print $query;
}
?>