Why do we get error "Call to undefined function: ora_logon()/ ocilogon()"?

PHP is not using the correct extension module. Try compiling PHP with the following options:

--with-oracle=/path/to/oracle/home/dir
--with-oci8=/path/to/oracle/home/dir

On Windows systems one can just uncomment the following lines in the php.ini file:

;extension = php_oci8.dll
;extension = php_oracle.dll