The SitePoint PHP blog has a post that asks developers "Which reference sites do you trust?"

While completing the tech edit on the 2nd edition of the PHP Anthology the issue of linking came up; specifically, linking to authoritative reference material on the web. [...] The issue isn't simple one. Regarding PHP, the php.net manual is the authoritative reference site. The various web technology standards are also represented by authority web sites.

So what does he thing makes for a good, trustworthy resource? He includes criteria divided up into three sections - readability, accuracy & trustworthiness and longevity.