Thanks Sirdude, but I find it a sound cynical to refer to the manual and quote the isset explanation. Especially since that doesn't answer the question.

I do use my php_manual.chm very often and did so too when I ran into this code. I just never was able to find a construct like the one used, hence my question.

It's one thing to know where to look (php manual and many websites) but it's another thing to know what to look for. Not isset that's for sure...
This morning I woke up and realised I had probably been looking in the wrong place all the time.

Just like Allen did, I too looked for isset, while in fact I should have looked at the && evaluation. And indeed there it's explained perfectly.

I do now understand that this code:
PHP Code
isset($user['USER_AVATAR']) && $Picture = $user['USER_AVATAR'];
if (!isset($Picture)) { $Picture = "http://"; }

does exactly the same as this code:
PHP Code
if (isset($user['USER_AVATAR'])) $Picture = $user['USER_AVATAR'];
if (!isset($Picture)) { $Picture = "http://"; }

But why would one opt for 2 different styles of evaluating an isset in 2 consecutive lines?
That still puzzles me.