What does the comma mean in an echo expression?

Question

I am trying to repeat a line from the Recursive function:
echo "<li>", $node, recurse($arr), "</li>";
and
echo "<li>" . $node . recurse($arr) . "</li>";

function writeList($tree)
{
    if($tree == null) return;
    echo "<ul>";
    foreach($tree as $node=>$children) {
        echo "<li>", $node, writeList($children) , "</li>";
    }
    echo "</ul>";
}

$treeis a tree structure as can be found in this question (form2)

And, I can notice that the result is from two different.
Can someone tell me the difference in use ,and .in general and, in particular, in this example?

EDIT : what if, instead of repeating the lines, I want to save the line generated from this function in a variable. I am particularly interested in the result obtained from the first operator echo.

EDIT: I feed on this array:

array
  3 => 
    array
      4 => 
        array
          7 => null
          8 => 
            array
              9 => null
      5 => null
  6 => null

The outputs I get:
(from the first echo expression)

<ul><li>3<ul><li>4<ul><li>7</li><li>8<ul><li>9</li></ul></li></ul></li><li>5</li></ul></li><li>6</li></ul>

(from the second expression of the echo)

<ul><ul><ul><li>7</li><ul><li>9</li></ul><li>8</li></ul><li>4</li><li>5</li></ul><li>3</li><li>6</li></ul>

Answer 1

EDIT: , . writeList(). echo.

:

echo "<li>", $node, writeList($arr), "</li>";

, . :

echo "<li>";
echo $node;
echo writeList($arr);
echo "</li>";

:

echo "<li>" . $node . writeList($arr) . "</li>";

., . , writeList($arr) , then echo.

, . , , , echo .

---

, , , , . , , , .

.

ob_start();
echo "<li>", $node, writeList($arr), "</li>";
$out = ob_get_clean();

, ?

$out = "<li>" . $node . writeList($arr) . "</li>";

Answer 2

echo - , . .

- , .

: , . , echo() es stuff. <li> $node ; .

WriteList return .