According to the php.net manual pages, http://php.net/manual/en/function.curl-setopt.php, the following is stated:
CURLOPT_SSLVERSION One of CURL_SSLVERSION_DEFAULT (0), CURL_SSLVERSION_TLSv1 (1), CURL_SSLVERSION_SSLv2 (2), CURL_SSLVERSION_SSLv3 (3), CURL_SSLVERSION_TLSv1_0 (4), CURL_SSLVERSION_TLSv1_1 (5) or CURL_SSLVERSION_TLSv1_2 (6).
Note: Your best bet is to not set this and let it use the default. Setting it to 2 or 3 is very dangerous given the known vulnerabilities in SSLv2 and SSLv3.
How can I set CURLOPT_SSLVERSION globally to CURL_SSLVERSION_TLSv1.