http://*.com/questions/22478387/call-to-undefined-function-oci-connect

1. Whenever you connecting Oracle Database , try to use 32 Bit oracle client libraries, Since XAMP PHP is compiled with 32 Bit(Though you have 64 Bit windows Machine)

2. Download Oracle Client from Download From here

3. Paste it in C:\instantclient_12_1

4. Then Set the path to above in System Environment Variable
5. Then Go to C:\xampp\php\php.ini and uncomment extension=php_oci8_12c.dll
6. Then Restart the XAMP and it should work without any Issue.