Change WooCommerce add to cart quantity into a drop-down

woo-quantity-dropdown.php

<?php 
/**
  * Output the quantity input for add to cart forms.
  *
  * @param  array           $args Args for the input.
  * @param  WC_Product|null $product Product.
  * @param  boolean         $echo Whether to return or echo|string.
  *
  * @return string
  */
 function woocommerce_quantity_input( $args = array(), $product = null, $echo = true ) {
   if( is_null( $product ) ) $product = $GLOBALS['product'];

   $defaults = array(
     'input_id'     => uniqid( 'quantity_' ),
     'input_name'   => 'quantity',
     'input_value'  => '1',
     'classes'      => apply_filters( 'woocommerce_quantity_input_classes', array(), $product ),
     'max_value'    => apply_filters( 'woocommerce_quantity_input_max', -1, $product ),
     'min_value'    => apply_filters( 'woocommerce_quantity_input_min', 0, $product ),
     'step'         => apply_filters( 'woocommerce_quantity_input_step', 1, $product ),
     'pattern'      => apply_filters( 'woocommerce_quantity_input_pattern', has_filter( 'woocommerce_stock_amount', 'intval' ) ? '[0-9]*' : '' ),
     'product_name' => $product ? $product->get_title() : ''
   );

   $args = apply_filters( 'woocommerce_quantity_input_args', wp_parse_args( $args, $defaults ), $product );

   // Apply sanity to min/max args - min cannot be lower than 0.
   $args['min_value'] = max( $args['min_value'], 0 );
   $args['max_value'] = 0 < $args['max_value'] ? $args['max_value'] : '';

   $extra_option = false;
   $def_max_val  = 20;
   if ( $args['max_value'] === '' || $args['max_value'] > $def_max_val ){
     $args['max_value'] = $def_max_val;//default max value if not defined
     if ( $args['input_value'] > $args['max_value'] ) $extra_option = $args['input_value'];
   }

   // Max cannot be lower than min if defined.
   if ( '' !== $args['max_value'] && $args['max_value'] < $args['min_value'] ) {
     $args['max_value'] = $args['min_value'];
   }

   if ( $args['max_value'] && $args['min_value'] === $args['max_value'] ) {
     ?>
     <div class="quantity hidden">
       <input type="hidden" id="<?php echo esc_attr( $args['input_id'] ); ?>" class="qty" name="<?php echo esc_attr( $args['input_name'] ); ?>" value="<?php echo esc_attr( $args['min_value'] ); ?>" />
     </div>
     <?php
   } else {
     /* translators: %s: Quantity. */
     $label = ! empty( $args['product_name'] ) ? sprintf( __( '%s quantity', 'woocommerce' ), wp_strip_all_tags( $args['product_name'] ) ) : __( 'Quantity', 'woocommerce' );
     ?>

     <div class="quantity">
       <label class="screen-reader-text" for="<?php echo esc_attr( $args['input_id'] ); ?>"><?php echo esc_attr( $label ); ?></label>

       <select name="<?php echo esc_attr( $args['input_name'] ); ?>"
         id="<?php echo esc_attr( $args['input_id'] ); ?>"
         class="<?php echo esc_attr( join( ' ', (array) $args['classes'] ) ); ?>"
         title="<?php echo esc_attr_x( 'Qty', 'Product quantity input tooltip', 'woocommerce' ); ?>">
         <?php
         for ( $count = $args['min_value']; $count <= $args['max_value']; $count = $count + $args['step'] ) {
           // Cart item quantity defined?
           if ( '' !== $args['input_value'] && $args['input_value'] >= 1 && $count == $args['input_value'] ) {
             $selected = 'selected';
           } else $selected = '';

           echo '<option value="' . $count . '"' . $selected . '>' . $count . '</option>';
         }
         if( $extra_option != false ) echo '<option value="'.$extra_option.'" selected>'.$extra_option.'</option>';
         ?>
       </select>

     </div>

     <?php
   }

 }