Description:
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When using http_build_query, passing an empty 3rd parameter results in no separator being used in the resulting output.

This is incorrect, as the documentation page specifies that 'arg_separator.output' will be used in the event that the 3rd parameter is empty.

To fix this issue in my scripts, I've had to explicitly define the separator to use, using:
$sep = ini_get('arg_separator.output');

Reproduce code:
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<?php

$query_string = 'mode=order_message&orderids=5048';
parse_str(html_entity_decode($qs), $query_string);

$var = http_build_query($query_string);

echo $var;

?>

Expected result:
----------------
'mode=order_message&orderids=5048'

Actual result:
--------------
'mode=order_messageorderids%3D5048'