php_gd.dll not found in php.ini !!

I installed PHP successfully on my Ubuntu 10.04

I checked the installation with phpinfo() and saw that the GD has been installed successfully :

GD Support enabled

GD Version 2.0

FreeType Support enabled

FreeType Linkage with freetype

FreeType Version 2.3.11

T1Lib Support enabled

GIF Read Support enabled

GIF Create Support enabled

JPEG Support enabled

libJPEG Version 6b

PNG Support enabled

libPNG Version 1.2.42

WBMP Support enabled

but gd functions appears to be not enabled on my server, so I tried to change the ";extension=php_gd.dll" line in php.ini and remove the ";" but the php.ini did not contain this line!

What should I do now? Is there any missing part?

Please help me.

thanks