InDesign wouldn't be able to use any encoding specified in the header. (It wouldn't even see it, as it's not kept when you save to disc in Windows.) Instead you have to explicitly tell it the encoding in a special tag of its own at the start of the file, such as:
Unfortunately, it does not use standard encoding names and there is no tag that InDesign understands that corresponds to UTF-8 encoding at all. The only encoding tag you can use that will allow you to include any character you like is:
which corresponds to UTF-16 (little-endian with BOM), with Windows CRLF line endings. (The only other line ending option is MAC, which you don't want at all as it's old-school pre-OSX Macs where the line ending character was CR.)
So, given a UTF-8 string $s including UTF-8 byte sequences you've pulled out of the database and plain (Unix-Linux-OSX-web-style) LF newlines, you'd write it like this:
$s= "<UNICODE-WIN> ".str_replace(" ", " ", $s); echo iconv('UTF-8', 'UTF-16', $s);
(Ensuring not to output any whitespace before or after, because it'll break the UTF-16 encoding.