There are some good examples online. Simply by googling "php google charts" you can find many examples of integrating PHP and Google Charts ie: https://developers.google.com/chart/interactive/docs/php_example
Based on that above example , it is using a PHP function on the server side to output a JSON document that gets sent to the browser for rendering with some JS (Google Viz API)
So in a Splunk context , on the server side , you'd execute a Splunk search using the Splunk SDK for PHP and output the search results in JSON. Code examples of executing a Splunk search are here.
