This pecl .dll is in use:

https://pecl.php.net/package/oci8/2.2.0/windows
oci8 2.2.0 for Windows
7.4 Thread Safe (TS) x64

Use deplister.exe (contained in all PHP downloads from
windows.php.net) to check for the dependencies of php_oci8_12.dll:

    deplister php_oci8_12.dll

I assume that OCI.dll is not in your PATH.

Hmm, I wonder what's the difference between ext/oci8 and
PECL/oci8.

Don't now - deplister show nothing. Maybe I have to ask my admin?
---------

X:\AERUS\php-7.4.7-Win32-vc15-x64>deplister.exe ext\php_oci8_12.dll

X:\AERUS\php-7.4.7-Win32-vc15-x64>pause
Drücken Sie eine beliebige Taste . . .

-----------------


Look on my phpinfo() - Apache Environment - Path:
------------
C:\oracle\product\11.2.0\client_1\bin;C:\oracle\product\11.2.0\client_1;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Windows\system32\config\systemprofile\AppData\Local\Microsoft\WindowsApps

phpinfo();
PDO_OCI is loaded well.

> Don't now - deplister show nothing.

If the DLL doesn't exist, deplister shows nothing for me as well.
Otherwise I get (regardless of having the orinally delivered DLL
or the PECL one):

D:\Users\cmb\Downloads\php-7.4.7-Win32-vc15-x64>deplister ext\php_oci8_12c.dll
php7ts.dll,OK
OCI.dll,NOTFOUND
VCRUNTIME140.dll,OK
api-ms-win-crt-heap-l1-1-0.dll,OK
api-ms-win-crt-time-l1-1-0.dll,OK
api-ms-win-crt-string-l1-1-0.dll,OK
api-ms-win-crt-runtime-l1-1-0.dll,OK
api-ms-win-crt-stdio-l1-1-0.dll,OK
KERNEL32.dll,OK

> phpinfo();
> PDO_OCI is loaded well.

Then the error message should no longer occur.

There are more questions than answers. You are using php_oci8_12c.dll but only have Oracle version 11 in PATH.  You may need the correct VS redistributable.  There are a bunch of questions/answers best followed up outside the bug system.  Please ask on a forum such as https://www.oracle.com/database/technologies/appdev/php.html before we investigate as a bug.

There are more questions than answers. You are using php_oci8_12c.dll but only have Oracle version 11 in PATH.  You may need the correct VS redistributable.  There are a bunch of questions/answers best followed up outside the bug system.  Please ask on a forum such as https://www.oracle.com/database/technologies/appdev/php.html before we investigate as a bug.

Ok, now I have checked these two DLLs with deplister.exe. Both find the OCI.DLL and the error in Apache error.og still exists.

[1. Test]--------------------------------------------------
deplister ext\php_oci8_11g.dll
C:\Program Files\AERUS\php-7.4.7-Win32-vc15-x64>deplister ext\php_oci8_11g.dll
php7ts.dll,OK
OCI.dll,OK
VCRUNTIME140.dll,OK
api-ms-win-crt-heap-l1-1-0.dll,NOTFOUND
api-ms-win-crt-time-l1-1-0.dll,NOTFOUND
api-ms-win-crt-string-l1-1-0.dll,NOTFOUND
api-ms-win-crt-runtime-l1-1-0.dll,NOTFOUND
api-ms-win-crt-stdio-l1-1-0.dll,NOTFOUND
KERNEL32.dll,OK

[2. Test]--------------------------------------------------
deplister ext\php_oci8_12c.dll

C:\Program Files\AERUS\php-7.4.7-Win32-vc15-x64>deplister ext\php_oci8_12c.dll
php7ts.dll,OK
OCI.dll,OK
VCRUNTIME140.dll,OK
api-ms-win-crt-heap-l1-1-0.dll,NOTFOUND
api-ms-win-crt-time-l1-1-0.dll,NOTFOUND
api-ms-win-crt-string-l1-1-0.dll,NOTFOUND
api-ms-win-crt-runtime-l1-1-0.dll,NOTFOUND
api-ms-win-crt-stdio-l1-1-0.dll,NOTFOUND
KERNEL32.dll,OK

C:\Program Files\AERUS\php-7.4.7-Win32-vc15-x64>dir


[php.ini]------------------------------------
php.ini changed to php_oci8_11g

; 03.07.2020 TW
;extension=php_oci8_12c 
extension=php_oci8_11g  

[Apache Error.log]------------
PHP Warning:  PHP Startup: Failed to load c:/Program Files/AERUS/php-7.4.7-Win32-vc15-x64/ext\\php_oci8_11g, Das System kann die angegebene Datei nicht finden.\r\n in Unknown on line 0


[phpinfo]---------------
phpinfo();
don't listed OCI
listed PDO_OCI

> api-ms-win-crt-heap-l1-1-0.dll,NOTFOUND
> api-ms-win-crt-time-l1-1-0.dll,NOTFOUND
> api-ms-win-crt-string-l1-1-0.dll,NOTFOUND
> api-ms-win-crt-runtime-l1-1-0.dll,NOTFOUND
> api-ms-win-crt-stdio-l1-1-0.dll,NOTFOUND

That's strange, because these are needed anyway.  You may need to
update the Visual C++ Redistributable for Visual Studio.

Anyhow, I don't think there is a bug regarding PHP/PECL, so you're
better off asking in a support forum.  Thanks.

>> phpinfo();
>> PDO_OCI is loaded well.
>
>Then the error message should no longer occur.

Not really. php_pdo_oci.dll is fairly tolerant. For instance, it loads OK with a too old OCI.dll, while php_oci8_12c.dll exits with a missing procedure:

PHP Warning:  PHP Startup: Unable to load dynamic library 'oci8_12c' (tried: ext\oci8_12c (The specified module could not be found.), ext\php_oci8_12c.dll (The specified procedure could not be found.)) in Unknown on line 0

There seems to be no difference between the PHP- and the PECL-versions, allthough they differ quite a lot in size (150 KB vs 158 KB).

Anyway, (re)installing the VC++ Redistributable seems at least required: https://aka.ms/vs/16/release/VC_redist.x64.exe

> Die angegebene Prozedur wurde nicht gefunden.

translates to 

> The specified procedure could not be found.

Which indicates a too old OCI.dll. Looling at the other comment it is a 11g OCI.dll while php_oci8_12c.dll requires a 12c OCI.dll.

Thank you for your help, tips and information. The problem seems to be very specific and is related to Windows Server 2016. This is a newly set up server. The update file VC_redist.x64.exe I have installed and repaired in the meantime (several times). The missing api-ms-win-crt-*.dlls are still missing.