Imagepng's Strange questions about

Strange problems with Imagepng
The code below, it is strange to change imagepng ($im) to Imagepng ($im, "kk.png") can get the picture but the direct output to the browser is not displayed, has been a red fork, this code on another machine can run normally. Is my PHP configuration problem?
Header ("Content-type:image/png");
Srand (Double) microtime () *1000000);
$login _check_number=strval (rand ("11111", "99999"));
$session->setmember ("Loginchecknumber", $login _check_number);

$im = Imagecreate (50,20) or Die ("Cannot Initialize new GD image Stream");
$black = Imagecolorallocate ($im, 9,142,204);
$foreColor = Imagecolorallocate ($im, 0,0,0);
$white = Imagecolorallocate ($im, 255,255,255);
Imageline ($im, 1, 1, $black);
Imagearc ($im, Max, N, a, A, a, $white);
Imagestring ($im, 5, 7, 3, $login _check_number, $white);
for ($i =0; $i < $i + +)//Add interfering pixels
{
Imagesetpixel ($im, Rand ()%50, Rand ()%50, $foreColor);
}

Imagepng ($im);
Imagedestroy ($im);
?>


------Solution--------------------
discuss
Reference:
Reference:
GD Library does not open words imagepng ($im, "kk.png") can generate Kk.png file?

It's supposed to be about the browser.
Replace the first sentence with a trial header ("Content-type:image/x-png");


It's not going to change.

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