PHP Startup: Unable to load dynamic library

PHP Startup: Unable to load dynamic library

I have Apache 2.2 installed on my XP machine. Today I installed also PHP.
When I start Apahce, it gives me the error
PHP Startup: Unable to load dynamic library 'C:\Program
Files\PHP\ext\php_[extension].dll'
for several extensions (curl, mysql and ldap, the others are working).
What is weird is that the dll files are actually there.
php.ini:
extension_dir = "C:\Program Files\PHP\ext"
extension=php_bz2.dll
extension=php_curl.dll
extension=php_gd2.dll
extension=php_id3.dll
extension=php_imagick.dll
extension=php_ldap.dll
extension=php_mbstring.dll
extension=php_mssql.dll
extension=php_mysql.dll
extension=php_zip.dll
What can be the cause ?