如何获得“选择”值并输入“输入”值？

I have a for with a select dropdown option. I want to get the value that is selected and put it into a hidden input value. I'm trying this at the minute -

echo '<form method="post" action="cart_update.php">';
echo"<select name='qty'>
             <option value=1>1</option>
             <option value=2>2</option>
             <option value=3>3</option>
             <option value=4>4</option>
             <option value=5>5</option>
             <option value=6>6</option>
             <option value=7>7</option>
             <option value=8>8</option>
             <option value=9>9</option>
             <option value=10>10</option>
             </select>";
echo '<input type="hidden" name="product_qty" value="'.$_GET['qty'].'" />';
echo"</form>;

When I submit this to cart-update.php it tells me that qty is an undefined index on the hidden input line.

Can anyone see the problem?

A few problems: 1. Your form is in POST method, and you are trying to get the value in "Get" method ($_GET) 2. The value of $_GET will be available only after submitting the form.

You should use javascript to get the value. something like that:

<form method="post" action="cart_update.php">
<select name='qty' onChange='document.getElementById("product_qty").value = this.value'>
    <option value=1>1</option>
    <option value=2>2</option>
    <option value=3>3</option>
    <option value=4>4</option>
    <option value=5>5</option>
    <option value=6>6</option>
    <option value=7>7</option>
    <option value=8>8</option>
    <option value=9>9</option>
    <option value=10>10</option>
</select>
<input type="hidden" id="product_qty" name="product_qty" value="1" />

you will have to use javascript or jQuery

sample jquery code:

$('select').change(function(){
var val1 = $('select').val();
$('your selector').val(val1);
});

Consider this instead:

$arr = array(1,2,3,4,5,6,7,8,9,10);
foreach($arr as $n){
  $selected = $n == $_POST['qty'] ? 'selected' : '';
  echo "<option value='$n' $selected>$n</option>";
}

Based on @Undefined_variable answer:

/***************************/
jQuery( document ).ready(function( $ ) {

/* selecy key from list , insert into field */ 

$('#select_start_key').change(function(){

var val1 = $('#select_start_key').val();

$('#start_key').val(val1);

/*reset the select to none*/
$('#select_start_key').val("");

});

});

View example :

https://jsfiddle.net/alinrzv/aj619Lbb/