How to get SEO-relevant site statistics using PHP?

My goal is to get various SEO-relevant in开发者_C百科formation for various sites. Basically all the stuff you see in SEO toolbars for your browsers (Page Rank, amount of links, position in search results, etc.).

But what's the best way to get this information? Where do these toolbars get it? Do they have their own indexing scripts and databases, or are there public databases with all this information that can be 'ripped' from some site, or requested through an API?

I need this for a rather large amount of sites, and also periodically for e.g. week over week comparisons, so doing this manually isn't really an option.

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They are most likely either scraping search results, or querying Google's API.