Please Help! trying to get sum

<?

$number = 3;
 
	echo "enter 3 numbers into the areas below, how can i make 
	it so when someone hits enter it adds up the three numbers?";
	echo "<form action="?op=do" method="post">";
	for($i=0;$i<$number; $i++) { 
	echo "<input type='text' name='field[$i]' size='4'> ";
	echo "<input type='hidden' name='number' value='$number'><br> ";
	} 
	echo "<INPUT type=submit value="enter">";
	echo "</form>";

if($op=="do"){

	for($i=0; $i<$number; $i++) {
	echo "$field[$i]";
	}
}

?>

Original - php Code <? $number = 3; echo "enter 3 numbers into the areas below"; echo "<form action="?op=do" method="post">"; for($i=0;$i<$number; $i++) { echo "<input type='text' name='field[$i]' size='4'> "; } echo "<INPUT type=submit value="enter">"; echo "</form>"; if($op=="do") { $sum = 0; for($i=0; $i<$number; $i++) { $sum += ($field[$i] + 0); } echo "$sum"; } ?>

 
<?
    $number = 3;
    echo "enter 3 numbers into the areas below";
    echo "<form action="?op=do" method="post">";
    for($i=0;$i<$number; $i++) {   
       echo "<input type='text' name='field[$i]' size='4'> ";
    } 
    echo "<INPUT type=submit value="enter">";
    echo "</form>";
    if($op=="do") {    
       $sum = 0;
       for($i=0; $i<$number; $i++) {
          $sum += ($field[$i] + 0);  
       }
       echo "$sum";
    }
 ?>