Ask why JavaScript does not pop up the box, the content is as follows

Why does JavaScript not pop up the prompt box, the content is as follows
(1)
$serial _number=1;
echo "";
?>
(2)
$serial _number=1;
echo "";
?>

(1) Frame output: 1
(2) No results, no prompt error
Excuse me, what is the reason, how to solve?
------Solution--------------------
Oh, I was wrong, you put alert ("I Am" +a+ "friend") in the "" Change to "
------Solution--------------------

echo "";

------Solution--------------------

$serial _number = ' Wang gang ';
echo "";

String constants are also quoted in JS, otherwise they are considered variables

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