I finally found a solution:
I downloaded the actual OCI-DLLs from "PECL :: PAckage :: oci8 :: 2.0.8" (in my case the 32bit, threadsafe DLL) and then removed the
; from this line
;extension=php_oci8_11g.dll ; Use with Oracle 11gR2 Instant Client
I also added the following lines to my http.conf of apache
SetEnv ORACLE_BASE "C:/oracle/" SetEnv ORACLE_HOME "C:/oracle/product/11.2.0/client_1"
After that I restarted apache, made sure that my connection-alias is probably placed in my
tsnames.ora and then I could use the oci8-functions witout any problems.