You need to build phalcon seperate for php 5.6 and php 7.1, the same extension can't work for both versions, first try to write php -v and check on which version you currently are. If you are on php 7 then you just need use zephir for building it and phalcon 2.1.x
Zephir build will automatically install extnesion to php lib folder.
upon starting php-fpm 5.6, i got the notice message "undefined symbol: phppdogetdbhce in Unknown on line 0", but when looking through phpinfo, the module was loaded
You need to load pdo BEFORE phalcon in this case.
when starting php-fpm 7.1 i get the message "undefined symbol: zendstdcallusercall in Unknown on line 0"
Here you have error beacause you copied phalcon from php 5.6 to php 7.1
If you want have both php versions at once then i would recommend just using phpbrew https://github.com/phpbrew/phpbrew