An important hint: if you are using prebuilt PHP packages where PDO_OCI is not compiled in, you should download the original PHP sources for the PHP version your server is using.
These sources contain the PDO_OCI source in the ext/pdo_oci directory.
Change to that directory, build the module using 
phpize
./configure
make
and then install the newly build module to your PHP extension directory and add it to your configuration.
For me this has worked on a Debian server with the sury.org modules (PHP 8.2).

if oracle and oracle instant client has been installed,
without db in the same host

For UNIX/LINUX，set $LD_LIBRARY_PATH
appent your instant client path  and  client/lib path to it,

For windows set PATH like this

After set the path ,set TNS_ADMIN everioment ,point to 
where  tnsnames.ora located.

Then,you can use service name to connect to your Database

Test coding

<?php
$param = $_POST;
$db_username = "youusername";
$db_password = "yourpassword";
$db = "oci:dbname=yoursid";
$conn = new PDO($db,$db_username,$db_password);
$name = $param['module'];
$file = $param['file'];
$stmt = $conn->exec("INSERT INTO AL_MODULE (AL_MODULENAME, AL_MODULEFILE) VALUES ('$name', '$file')");

?>

If you're getting the "I'm too dumb to find oci.h" error, try creating a variety of paths.  One variety uses just the major and minor of your OIC version (eg, 11.2 for 11.2.0.2) and another variety uses client64 as well as client.

Something like this (for 11.2.0.2):
ln -s /usr/include/oracle/11.2.0.2/ /usr/include/oracle/11.2
ln -s /usr/include/oracle/11.2/client /usr/include/oracle/11.2/client64
ln -s /usr/lib/oracle/11.2.0.2/ /usr/lib/oracle/11.2
ln -s /usr/lib/oracle/11.2/client /usr/lib/oracle/11.2/client64

This should cover your bases for 64-bit systems, as well as PHP patched to use the major.minor version number only.  See also PHP bug #44989.

To enable PDO support on PHP for Oracle Instant Client 11.1.x, you should follow the syntax above in the compile command, just as pointed by Andrew http://bugs.php.net/bug.php?id=39312, taking by default you have installed the OIC at /usr/lib/oracle (instant client and sdk at subfolder):

./configure --with-oci8=shared,instantclient,/usr/lib/oracle
--with-pdo-oci=instantclient,/usr/lib/oracle,11.1

Just saying your release version from the Oracle OIC.

It compiles fine then.

Best regards.

As addition to what I wrote 2 years ago:
if you are installing on Debian with a Debian provided package, install the source with 
apt source php8.2
Then you have to set ORACLE_HOME like that:
ORACLE_HOME=/usr/lib/oracle/19.6
export ORACLE_HOME
before running ./configure.
After that, I had to add the include directory 
-I/usr/include/oracle/19.6/client64/
into the Makefile to successfully build the module.

Take note of the note at the top, this really is an experimental extension. I had a problem trying to read data from Oracle which resulted in some strange behaviour in PHP. i.e. foreach loops not ending, with no error messages. I also managed to get the data from Oracle into an array in PHP, but then couldn't return the array from a function.

After pulling my hair out for a day, it turned out to be a CLOB column in Oracle that caused the strange behaviour in PHP. I assume this extension doesn't fully support them.

Instead I've typecast it within the SQL to a VARCHAR2 which seems to resolve it:

SELECT CAST(columnx AS VARCHAR2(4000)) AS columnx ...

It might help someone else having similar issues.

A Statement of Warning:

PDO::oci does not support REF CURSORS.

This is mentioned nowhere (until now!) on this page. 

And now you know!

If you want ref cursors avoid PDO for now. 

My Reference for this claim:
http://www.oracle.com/technology/pub/articles/
php_experts/otn_pdo_oracle5.html

GREAT article, excellent piece, really. It's not clear to me 
how old this document is, but it must have some dust on it, 
given it's references to "PHP5.1 ...' which is a little way off yet' "
... as of 2006-06-01, PHP5.1 has been with us for quite some time.

Notice the red block at the beginning of this page... pdo_oci is HIGHLY experimental.

Even though it is under dev from 2004, it lakes today support for things that _do_ matters :
 - bind a varchar2 of 3500 chars 
 - get selected metas
 - left join with blobs/clobs
 - etc.

For the story, since we use pdo_pgsql in our software, I thought it would be viable to use pdo_oci for running under Oracle. After a long battle, I finally won :

1) If requested driver has a non-experimental pdo version available, use it.
2) else (well, for pdo_oci at least), use an abstraction layer of your own.
3) you're done.

What I did in more details...

2 "main" classes for being compliant with "$obj instanceof PDO" or such :
 - class PhpDb extends PDO
 - class PhpDbStatement extends PDOStatement
2 "abstract" classes that defines what PDO actually does :
 - abstract class PhpDbAbstract
 - abstract class PhpDbAbstractStatement
And at last for each driver, 2 classes doing the abstraction :
 - class PhpDbDriverOracle extends PhpDbAbstract
 - class PhpDbDriverOracleStatement extends PhpDbAbstractStatement

"main" classes are accessed from your script, simply replace "new PDO" with "new PhpDb".
"abstract" classes are mainly there for the documentation :p
"driver" classes do the job in background, they are instances by the main classes.

My PhpDb will be in an open source project sooner or later, search google or mail me !