Download Oracle Instant Client libraries (URL may change, already happened few times). Be sure you download correct (x64 or x32) architecture. And correct version. I'm using Instant Client 10.1.0.5.0 with Oracle 10g, 11g and 12c and PHP 5.6, 7.1, 7.2, 7.3 and 7.4. Never hit any problem with such setup but my queries are quite simple.
Extract libraries into directory you prefer. I'm using /usr/local/lib/oracle. File tree follows:
/usr/local/lib/oracle/
/instantclient-10.1.0.5.0/
/sdk/
/libclntsh.so -> libclntsh.so.10.1
/libclntsh.so.10.1