Yar Concurrent Client doesn't work on Mac OS. It is because the cURL shipped with Mac has some difference.
you can resolve that by install a new cURL, like (brew install curl).
then use :
"export DYLD_LIBRARY_PATH=/path-to-new-curl/lib:$DYLD_LIBRARY_PATH"
make the PHP use the new cURL.
of course you can also re-compile your PHP with --with-curl=/path-to-new-curl/