First, contact support@advantagecom to make sure you are using the latest application template version of PHP available for your VPS.
If you are using the latest application template available for PHP and still wish to install a custom version of PHP instead first uninstall the HSPC templated version of PHP. This is necessary so that a future update of the PHP application template does not overwrite your custom installation of PHP.
Next make a server-side backup of the VPS prior to installing PHP.
Locate the version of PHP you wish to use online and download the source to your VPS.
Then compile PHP yourself from the source with the options you need, using the apxs binary supplied on your VPS account to ensure the new PHP will work with the included Apache.
It is recommended that you install the development tools application template to provide the tools for compiling applications. Alternately, you can upload your own compiler to your VPS to compile PHP.