root@aa553164e3bf:/var/www/xmlrpc# ls -la /proc/self/fd/1 
lrwx------ 1 root root 64 May 28 15:02 /proc/self/fd/1 -> /dev/pts/1
root@aa553164e3bf:/var/www/xmlrpc# ls -la /dev/pts/1 
crw--w---- 1 root tty 136, 1 May 28 15:02 /dev/pts/1

fopen('php://stdout', 'w')

Linux 63f6b1c48b39 4.15.0-50-generic #54-Ubuntu SMP Mon May 6 18:46:08 UTC 2019 x86_64

root@aa553164e3bf:/var/www/xmlrpc# ls -la /dev/st*
lrwxrwxrwx 1 root root 15 May 28 14:52 /dev/stderr -> /proc/self/fd/2
lrwxrwxrwx 1 root root 15 May 28 14:52 /dev/stdin -> /proc/self/fd/0
lrwxrwxrwx 1 root root 15 May 28 14:52 /dev/stdout -> /proc/self/fd/1

RUN set -eux ; \
    cd /app/log/ ; \
    touch app.log ;\
    ln -sfT /dev/stdout app.log ; \
   chown -R --no-dereference www-data:www-data . ;

root@63f6b1c48b39:/app# ls -la /var/log/apache2/
total 8
drwxrwxrwx 2 www-data www-data 4096 May  8 02:37 .
drwxr-xr-x 1 root     root     4096 May  8 02:37 ..
lrwxrwxrwx 1 www-data www-data   11 May  8 02:37 access.log -> /dev/stdout
lrwxrwxrwx 1 www-data www-data   11 May  8 02:37 error.log -> /dev/stderr
lrwxrwxrwx 1 www-data www-data   11 May  8 02:37 other_vhosts_access.log -> /dev/stdout
root@63f6b1c48b39:/app# ls -la log/
total 8
drwxr-xr-x 1 www-data www-data 4096 May 28 13:55 .
drwxr-xr-x 1 root     root     4096 May 28 13:55 ..
lrwxrwxrwx 1 www-data www-data   11 May 28 13:55 app.log -> /dev/stdout

 if ($f = fopen($logfile, 'a')) {
    fwrite($f, strftime("%Y/%m/%d - %H:%M:%S: ") . $data);
    fclose($f);
    return true;
} else {
   return false;
}

Warning:  fopen(/app/log/app.log): failed to open stream: Permission denied in ..