A simple PHP drawing related to problems,

A simple PHP drawing problem 、、、
I write the following code on the PHP page:
Header (' Content-type:text/jpeg '); This place can be executed correctly if it is annotated.
$im =imagecreatetruecolor (200,200);
Imagepng ($im);
?>

If the above place is not annotated, you cannot draw the picture. The browser also prompts to download (download is not a picture) Ah, or open (after opening is garbled).
Question: Does drawing have to have this header () to specify the type? Does it have something to do with PHP.ini's profile? Or the other reason, I can't think of it.

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You output the browser, you have to tell it, you want to export to others what kind of things??
Like HTML and the like is text/html ....
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I do not understand a bit, comment out the MIME type should be text/html, how to display the picture?
In addition, you output the PNG format, but the header sends the JPEG format. Let's see.
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Header (' content-type:image/png '); Note: image instead of text
$im =imagecreatetruecolor (200,200);
Imagepng ($im);

Red to match

The header of the type declaration is required, depending on the type of browser
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Header (' Content-type:image/jpeg '); Image is the specified picture type.
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Example 1. Create a new GD image stream and output the image

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