I have no idea how to access a PHP based WebService from a BlackBerry.
Can anybody guide me in accessing one?
Your options are either use a proper API interface as suggested by snowflake (like REST, JSON/XML RPC etc.) or to scrape the HTML (very painful -- especially on mobile devices). The language in which the service is implemented shouldn't matter. If you are targeting OS6, there is a new communication API that makes consuming webservices easier (A sample can be found at http://devblog.blackberry.com/2011/01/communication-api-2/), but it's not too difficult on OS5 or below either. If you need to use JSON in OS5, your can find the org.me.json package online and use that.
If the webservice is under your control, you should add RPC support, the difficulty of which depends on the framework (if any) you're using for your project. Your question's a little scant on that information, so I can't provide any specific details.
See this Web Services in BlackBerry J2ME Application article. Here clearly described how to use Web services in Blackberry. And also you can refer Web services overview documentation.