getTraceAsString（）返回值中的“{main}”是什么意思？

I'm consistently getting an additional stack frame when I look at an exception's trace with the getTraceAsString() method. It's neither visible in getTrace() nor in debug_backtrace(). Where does it come from and what does it mean?

For example, this script:

function foo () {
    echo "== debug_backtrace() ==
";
    foreach (debug_backtrace() as $i => $frame) {
        echo "#$i $frame[file]($frame[line]): $frame[function]()
";
    }
    throw new Exception();
}

function bar () {
    foo();
}

try {
    bar();
} catch (Exception $e) {
    echo "
== getTrace() ==
";
    foreach ($e->getTrace() as $i => $frame) {
        echo "#$i $frame[file]($frame[line]): $frame[function]()
";
    }
    echo "
== getTraceAsString() ==
";
    echo $e->getTraceAsString(), "
";
}

produces the following output:

== debug_backtrace() ==
#0 /home/foo/tests/php/test4.php(13): foo()
#1 /home/foo/tests/php/test4.php(17): bar()

== getTrace() ==
#0 /home/foo/tests/php/test4.php(13): foo()
#1 /home/foo/tests/php/test4.php(17): bar()

== getTraceAsString() ==
#0 /home/foo/tests/php/test4.php(13): foo()
#1 /home/foo/tests/php/test4.php(17): bar()
#2 {main}

The only other place where I've seen {main} was with Xdebug. I guess it's some internal PHP stack frame, but why is it only visible when getting the trace as a string?