error don’t hold value in increase and decrease quantity in php using loop

I have a problem, when I click to other -/+ in new row, the old row return initial value. How can I fix it?

<?php
$arr = array(1, 2, 3);
for ($i = 0; $i < 3; $i++) {
    $arr[$i] = isset($_POST["item".$i]) ? $_POST['item'.$i] : $arr[$i];
    if (isset($_POST['incqty'.$i])) {
        $arr[$i] += 1;
    }

    if (isset($_POST['decqty'.$i])) {
        $arr[$i] -= 1;
    }
}
?>
<?php

for ($i = 0; $i < 3; $i++) {

?>
    <form method='post' action='<?= $_SERVER['PHP_SELF']; ?>'>
        <td>
            <button name='decqty<?php echo $i; ?>'>-</button>
            <input type='text' size='1' name='item<?php echo $i; ?>' value='<?= $arr[$i]; ?>' />
            <button name='incqty<?php echo $i; ?>'>+</button>
        </td>
    </form>
<?php
 } 
 ?>

change value when end click other row