An idea on windows is that the library path isn't set in the %PATH% variable, or perhaps the 32bit and 64bit instant clients are conflicting, with respect to your php install
Here's what the PHP: Installation - Manual has to say on the matter:
Installing OCI8 on Windows
When using Oracle 10gR2 client libraries on Windows, uncomment the php.ini line extension=php_oci8.dll. When using Oracle 11gR2 or later client libraries, uncomment extension=php_oci8_11g.dll or extension=php_oci8.dll. With Oracle 12c libraries use extension=php_oci8_12c.dll or extension=php_oci8_11g.dll or extension=php_oci8.dll. Only one of these DLLs may be enabled at a time. DLLs with higher versions may contain more functionality. Not all DLLs may be available for all versions of PHP. Make sure extension_dir is set to the directory containing the PHP extension DLLs.
If using Instant Client, set the system PATH environment variable to the Oracle library directory.
I agree with Gaz in Oz. Only use one of the Instant Clients (with PHP 5 you'll almost certainly need the 32bit client). Check the Instant Client directory is in PATH. And uncomment (remove the semicolon) from the php.ini line