what is the purpose of the extra asterisks in php comments?

The /** stuff */ document is also referred to as DocBlock notation.

It's used to document PHP functions, classes, etc.

/** 
* A test function
*
* @param  foo $bar
* @return baz
*/
function test(foo $bar) { return new baz; }

Some editors make good use of this to perform their Code Insight feature, a very powerful tool that reduces the time you have to spend looking at your old function declarations. Aptana and Zend Studio have this feature, probably others as well.

You can also use it in combination with Reflection to do some sort of AOP, doing runtime inspection of the DocBlock above your declarations. In fact, Doctrine uses it to build an object attribute map for their "Active Record" implementation.

I agree with ajon and Quentin. Mainly it's readability. However,there is one more thing.

There are automatic documentation generators (like doxygen ).

They require some particular comment formatting to include these comments into documentation. I believe this style of commenting is used exactly for this purpose (look at following doxygen documentation page - http://www.doxygen.nl/manual/docblocks.html)

Readability.

It clearly highlights the commented section to people reading the code.

The double asterisk comment is used sometime by certain documentation systems. The documentation system will proccess the block and look for certain keywords like @author or @var.

Single asterisk comments wil be treated as // comments.

See PhpDoc http://www.phpdoc.org/docs/latest/guides/types.html