When including MantisBT Core for a command-line tool, the output is delayed until the buffer is full, or the script terminates.

To avoid this behavior, CLI script must end output buffering, twice

<?php
include 'core.php';
echo 'foo'; // not displayed
ob_end_flush(); ob_end_flush();  // prints 'foo'

There are indeed 2 calls to ob_start():

• core.php line 90. This was introduced in 0008761, apparently to avoid Content Encoding errors - not sure if this is still an issue today.
• compress_api.php in compress_start_handler() function

It's additionally worth noting that the ini_set( 'zlib.output_compression', true ); call in compress_handler_is_enabled() function, effectively also starts output buffering.