Description:
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Function imagejpeg() is not able to write to the user-defined stream.

[I think it is caused by the fact that _php_image_output_ctx() uses VCWD_FOPEN() instead of php_stream_open_wrapper() - but I'm not an expert.]

Reproduce code:
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imagejpeg(imagecreatefromjpeg("x.jpg"), "var://myvar");

"var" protocol defined as an example in http://www.php.net/stream_wrapper_register

Expected result:
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Image successfully copied to the variable.

Actual result:
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Warning: imagejpeg(): Unable to open 'var://myvar' for writing: Invalid argument