WooCommerce Dropdown Product Quantity, fully compatible with Min/Max quantities extension and working quantity

functions.php

<?php
// Place the following code in your theme's functions.php file
// override the quantity input with a dropdown
// Note that you still have to invoke this function like this:
/*
$product_quantity = woocommerce_quantity_input( array(
  'input_name'  => "cart[{$cart_item_key}][qty]",
  'input_value' => $cart_item['quantity'],
  'max_value'   => $_product->backorders_allowed() ? '' : $_product->get_stock_quantity(),
  'min_value'   => '0'
), $_product, false );
*/
function woocommerce_quantity_input($data) {
    global $product;

  $defaults = array(
    'input_name'    => $data['input_name'],
    'input_value'   => $data['input_value'],
    'max_value'   => apply_filters( 'woocommerce_quantity_input_max', '', $product ),
    'min_value'   => apply_filters( 'woocommerce_quantity_input_min', '', $product ),
    'step'    => apply_filters( 'woocommerce_quantity_input_step', '1', $product ),
    'style'   => apply_filters( 'woocommerce_quantity_style', 'float:left; margin-right:10px;', $product )
  );
  if ( ! empty( $defaults['min_value'] ) )
    $min = $defaults['min_value'];
  else $min = 1;

  if ( ! empty( $defaults['max_value'] ) )
    $max = $defaults['max_value'];
  else $max = 20;

  if ( ! empty( $defaults['step'] ) )
    $step = $defaults['step'];
  else $step = 1;

  $options = '';
  for ( $count = $min; $count <= $max; $count = $count+$step ) {
    $selected = $count === $defaults['input_value'] ? ' selected' : '';
    $options .= '<option value="' . $count . '"'.$selected.'>' . $count . '</option>';
  }
  echo '<div class="quantity_select" style="' . $defaults['style'] . '"><select name="' . esc_attr( $defaults['input_name'] ) . '" title="' . _x( 'Qty', 'Product quantity input tooltip', 'woocommerce' ) . '" class="qty">' . $options . '</select></div>';
}
?>

To get it working on both product pages and cart, you need to ensure the name attribute is set correctly.

if (!empty($defaults['input_name'])) {
  $name = esc_attr($defaults['input_name']);
} else {
  $name = 'quantity';
}

And then use this in your select element, mine is like so

<select name="' . $name . '" title="' . _x( 'Qty', 'Product quantity input tooltip', 'woocommerce' ) . '" class="qty">' . $options . '</select></div>';

/**---------------------------------------------------------------------------
 * Change quantity in to dropdown
 ----------------------------------------------------------------------------*/
function woocommerce_quantity_input($data = null) {
 global $product;
  if (!$data) {
    $defaults = array(
      'input_name'    => 'quantity',
      'input_value'   => '1',
      'max_value'     => apply_filters( 'woocommerce_quantity_input_max', '', $product ),
      'min_value'     => apply_filters( 'woocommerce_quantity_input_min', '', $product ),
      'step'          => apply_filters( 'woocommerce_quantity_input_step', '1', $product ),
      'style'         => apply_filters( 'woocommerce_quantity_style', 'float:left;', $product )
    );
  } else {
    $defaults = array(
      'input_name'    => $data['input_name'],
      'input_value'   => $data['input_value'],
      'max_value'     => apply_filters( 'woocommerce_quantity_input_max', '', $product ),
      'min_value'     => apply_filters( 'woocommerce_quantity_input_min', '', $product ),
      'step'          => apply_filters( 'woocommerce_quantity_input_step', '1', $product ),
      'style'         => apply_filters( 'woocommerce_quantity_style', 'float:left;', $product )
    );
  }
  
  if ( ! empty( $defaults['min_value'] ) )
    $min = $defaults['min_value'];
  else $min = 1;
  if ( ! empty( $defaults['max_value'] ) )
    $max = $defaults['max_value'];
  else $max = 100;
  if ( ! empty( $defaults['step'] ) )
    $step = $defaults['step'];
  else $step = 1;
  $options = '';
  //fix add to cart data to send on cart page
	if (!empty($defaults['input_name'])) {
	  $name = esc_attr($defaults['input_name']);
	} else {
	  $name = 'quantity';
	}
	
  for ( $count = $min; $count <= $max; $count = $count+$step ) {
    $selected = $count === $defaults['input_value'] ? ' selected' : '';
    $options .= '<option value="' . $count . '"'.$selected.'>' . $count . '</option>';
  }
  
  echo '<div class="quantity_select" style="' . $defaults['style'] . '"><select name="' . $name . '" title="' . _x( 'Qty', 'Product quantity input tooltip', 'woocommerce' ) . '" class="qty">' . $options . '</select></div>';
}