Sorry if this sounds like a really silly question.
But I Googled the web and also Googled specifically both the php.net site and the stackoverflow.com site.
This is not an uncommon feature of expression evaluation. The PHP manual page on logical operators makes a passing reference to it in one of the illustrative examples though.
Short circuit evaluation is a commonly exploited idiom, and you can rely on its continued support in the language, otherwise vast amounts of code would break!
Closest thing I can find to an 'official' mention of PHP's short-circuit implementation: http://php.net/manual/en/language.operators.logical.php