This affects both SpiceCRM and Kreporter in SuiteCRM.
When using a supported PHP (7.0) and apache2 + php-fpm (at least here on Debian Stretch) getallheaders() is not available as such KREST fails to work (blank screen).
To fix, somewhere in KREST/index.php (before Slim) add in this:
if (!function_exists('getallheaders'))
{
function getallheaders()
{
$headers = [];
foreach ($_SERVER as $name => $value)
{
if (substr($name, 0, 5) == 'HTTP_')
{
$headers[str_replace(' ', '-', ucwords(strtolower(str_replace('_', ' ', substr($name, 5)))))] = $value;
}
}
return $headers;
}
}