Если php_curl.dll есть в директории extensions ( а она там
должна быть), то проблема в том, что эта директория не просматривается Windows на предмет наличия в ней dll.
Читаем маны:
" ... You need to ensure that the dlls which php uses can be found. The precise
dlls involved depend on which web server you use and whether you want to
run php as a cgi or as a server module. php4ts.dll is always used. If you are
using a server module (e.g. isapi or apache) then you will need the relevent
dll from the sapi folder. If you are using any php extension dlls then you
will need those as well. To make sure that the dlls can be found, you can
either copy them to the system directory (e.g. winnt/system32 or
windows/system) or you can make sure that they live in the same directory
as the main php executable or dll your web server will use (e.g. php.exe,
php4apache.dll). "
Т.е. как вариант - скопировать dll из extensions в windows/sistem(32) каталог.
Успехов!
