>I wonder why there is no open source SharePoint alternative e.g. coded in PHP.

I think it's because SharePoint's strengths lie mainly in integration with Office and other Microsoft products. Lists, workflows, wikis, etc. in SharePoint aren't great. Basically everything it does can be done in a better way with another tool.

However, due to AD integration, Office integration, the ease of connecting to MS SQL Server, the product can do things that would be difficult to do in any third party product. You can upload Word documents, run a workflow on them, have users edit list data directly in Office products, bulk edit lists in Excel, and use InfoPath forms (basically dead now though from what I recall -- thankfully I no longer have to work with SharePoint).

I think SharePoint won't ever have a competitor that can offer all of its integration.