Well, I faced this error too and managed to fix it.
Before downloading every kind Oracle Client Version just take a look at your http://localhost/?phpinfo settings at Configure Command section.
This will tell, which version of Oracle was used to build your WAMP server.

The image above show this two lines:
"--with-pdo-oci=c:\php-sdk\oracle\x64\instantclient_12_1\sdk,shared"
"--with-oci8-12c=c:\php-sdk\oracle\x64\instantclient_12_1\sdk,shared"
Obviously tell us that InstantClient used is 64 bits.
There are two versions of InstantClient 12.1 x64: 12.1.0.2.0 and 12.1.0.1.0
I tested with 12.1.0.2.0 version and failed.
After that, I tested with old one, 12.1.0.1.0 version and worked.
Download from Instant Client for Microsoft Windows (x64) and extract the files below to "c:\oracle":
- instantclient-basic-windows.x64-12.1.0.2.0.zip
- instantclient-sqlplus-windows.x64-12.1.0.2.0.zip
- instantclient-sdk-windows.x64-12.1.0.2.0.zip
This will create the following folder "C:\Oracle\instantclient_12_1".
Using "Windows Command Prompt" (WIN+R CMD) go to folder above and execute SQLPLUS:
- sqlplus [user]/[pass]@[host]:[port]/[service]
You must be able to connect.
Finally, add the "C:\Oracle\instantclient_12_1" folder to the PATH enviroment variable, placing it on the leftmost place.
Close your WAMP server.
Yes close it, because the Enviroment Variable is only read at launch time.
So restarting the services may fail, leading you to a wrong result.
After restart, your PHPINFO would show the OCI8 and PDO_OCI loaded.

I hope it helps!