OK, this one has stumped me, and an hour of googling has gotten me no closer.

I have PHP 5.1.6 running on a WinXPro box with IIS 5.1. I went into php.ini and enabled the gd2 extension. But for some reason, phpinfo() in a web page still doesn't reflect the change:

ftp
FTP support enabled

hash
hash support enabled

However, if I do php -i from the command line, the changes are there:

ftp

FTP support => enabled

gd

GD Support => enabled
:
: snip
:
XBM Support => enabled

hash

hash support => enabled

There is only one php.ini on my system, in c:\php. The configuration path in the web-phpinfo points to the right place. The PHPRC environment variable is correct. There was no HKLM\Software\PHP\IniFilePath registry key, so I created it.

I've stopped and restarted IIS; stopped IIS, waited, and restarted; stopped, waited, killed any dllhost.exe processes, and restarted; stopped, waited, removed and recreated the application, and restarted; stopped, deleted and recreated the ISAPI filter, and restarted; but nothing seems to work. I've altered php.ini on this box before (enabling MSSQL, etc) without going through this mess.

PHP-CLI sees the change, which tells me its an IIS/ISAPI problem. What am I missing?

Try putting your php.in in c:\windows or wherever your windows is. I stopped fighting with php on my windows box when I put the .ini there.

1. As WebDevourer said, make sure that your php.ini in under %windir% or %windir%\system32

2. You need to restart IIS to reload the updated php.ini.

3. Make sure the anonymous IIS user has read access to php.ini, if your partition is NTFS.

I think I found the problem:
extension=php_mbstring.dll
was listed twice.

I guess it considered the php.ini file invalid and thus didn't apply it.