Ajouter un script externe au header sur Magento (http://jacques.sh/ajouter-un-script-externe-au-header-sur-magento.html)

core_text.xml

<block type="core/text" name="head.jquery">
    <action method="setText">
        <text><![CDATA[<script src="//ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>]]></text>
    </action>
</block>

create_block.php

<?php
$text = "Hello World!";
$block = Mage::app()->getLayout()->createBlock('core/text')->setText($text);

jquery.php

<?php
$text = '<script src="//ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>';
$block = Mage::app()->getLayout()->createBlock('core/text')->setText($text);

local.xml

<?xml version="1.0" encoding="utf-8" ?>
<layout version="0.1.0">

    <default>
        <reference name="head">
            <block type="core/text" name="head.jquery">
                <action method="setText">
                    <text><![CDATA[<script src="//ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>]]></text>
                </action>
            </block>
        </reference>
    </default>

</layout>

local_base_url.xml

<?xml version="1.0" encoding="utf-8" ?>
<layout version="0.1.0">

    <default>
        <reference name="head">
            <block type="core/text" name="head.base_url">
                <action method="setText">
                    <text><![CDATA[<script type="text/javascript">var BASE_URL = '{{baseUrl}}';</script>]]></text>
                </action>
            </block>
        </reference>
    </default>

</layout>