敬告
经过我多次测试，发现就算按照以下做法换了证书，一样会偶尔出现“Operation timed out”，多试几次就好了...

错误
sudo pecl search mbstring

Connection to `ssl://pecl.php.net:443‘ failed: Operation timed out

解决步骤
php -r "print_r(openssl_get_cert_locations());"

会得到类似的结果，找到default_cert_file的路径

Array

(

[default_cert_file] => /usr/local/php5/ssl/cert.pem

[default_cert_file_env] => SSL_CERT_FILE

[default_cert_dir] => /usr/local/php5/ssl/certs

[default_cert_dir_env] => SSL_CERT_DIR

[default_private_dir] => /usr/local/php5/ssl/private

[default_default_cert_area] => /usr/local/php5/ssl

[ini_cafile] => /usr/local/php5/ssl/cert.pem

[ini_capath] => 

)

记住需要更换的证书的路径为 /usr/local/php5/ssl/cert.pem
wget http://curl.haxx.se/ca/cacert.pem (wget下载不了的话，请用浏览器下载)
sudo mv cacert.pem /usr/local/php5/ssl/cert.pem
sudo pecl search mbstring
No releases available for package "pecl.php.net/mbstring"
完成，没有错误提示了，显示能连接，可惜无法搜索到mbstring扩展
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Connection to `ssl://pecl.php.net:443' failed: mac系统