问题Magento =插入元素并调用函数javascript

if u see javascript don't edit because its related with magento. thank you!

i have called a phtml script under price box in product view page with a validation function this is the phtml :

<script type="text/javascript">
//<![CDATA[

    $$('#product-price-<?php echo Mage::registry('current_product')->getId(); ?>').each (function(elem){
        $(elem).hide();
    });

    $('product-price-<?php echo Mage::registry('current_product')->getId(); ?>').show();
    var Cp = document.getElementById('product-price-<?php echo Mage::registry('current_product')->getId(); ?>');

    Cp.innerHTML = '$&nbsp;<input type="text" class="input-text price" name="custom_price" style="width:auto;" value="<?php echo Mage::registry('current_product')->getPrice(); ?>" onchange="onChangeCP(this);"/></span><input type="hidden" id="custom_price_total" name="custom_price_total" value="<?php echo Mage::registry('current_product')->getPrice(); ?>">';

    <?php 

        $product = Mage::getModel('catalog/product')->load(Mage::registry('current_product')->getId()); 
        $allowCustomPrice = $product->getAttributeText('allow_customprice');
        $minPrice = $product->getMinPrice(); //getAttributeText('min_price');
    ?>

    function changeCustomPrice(el){
        adviceBlock=$(el).up('span').down('.validation-advice');
        if(el.value.replace(',','.')>=<?php echo $minPrice; ?>){
            if(adviceBlock){
                adviceBlock.remove();
                el.removeClassName('validation-failed');
            }
        }else{
            if(!adviceBlock){
                el.insert({after:'<div class="validation-advice">The price should be equal or greater than <?php echo $minPrice; ?>.</div>'});
                el.addClassName('validation-failed');
            }
        }
        optionsPrice.productPrice=el.value;
        optionsPrice.reload();
    }
    optionsPrice.formatPrice=function(price){
        $('custom_price_total').value=price;
        return formatCurrency(price,optionsPrice.priceFormat);
    };
//]]>
</script>

this script hide the span that displays the price then it draw a input box with the price so that the client enter his price that should be greater than the mean price; if it s less then it draw an error.

when clicked in add to cart button the damn function doesn't execute and nothing is done !

if this method is wrong then how can i hide the span and add the input in product view using magento (php/...)