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?

Reply to discussion (solution)

The connection string in PHP is ".", not "+"

Oh, I was wrong, you put alert ("I Am" +a+ "friend") in the "" Change to "

echo "";

echo "";

Thank you, your answer is very correct, very carefully. But modify $serial_number= ' Wang Gang '; After the frame is not, how can i solve it?

Oh, I was wrong, you put alert ("I Am" +a+ "friend") in the "" Change to "

But modify $serial_number= ' Wang Gang '; After the frame is not, how can i solve it?

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

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

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

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

Well

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