php azure function can't set content type

Please provide a succinct description of the issue.

#### Repro steps

Provide the steps required to reproduce the problem

1. create a simple php-gd script that generates an image
2. use file_put_contents(getenv('res'), "content-type:image/png".$imagedata);

#### Expected behavior

content-type is set and content is parsed proproply

#### Actual behavior

parsed as json

#### Known workarounds

none i could find

#### Related information

Provide any related information

* Programming language used PHO
* Links to source