How to concat string and variable in Liquid Shopify in PHP with proxy URL

<script type="text/javascript">
{% if customer  %}
    {% assign customerid = '"<?php echo base_url('checkout?id='.$postdata['cart_id']); ?>' | append: '&customer=' | append: customer.id | append: '"' %}
{% else %}
    {% assign customerid = '"<?php echo base_url('checkout?id='.$postdata['cart_id']); ?>' | append: '"' %}
{% endif %}
var checkouturl = {{customerid}};
window.location.replace(checkouturl);
</script>