Creation of a new external module

I'm trying to create a new external module for PHP called acme. I used the ext_skel script to create the module directory and skeleton and then modified the config.m4 as indicated in that file. Then I moved to the created acme directory, issued the phpize command, configure and make. The compilation worked well and i found in ext/acme/modules the library acme.so. I installed in my PHP extension dir and the used the dl php command to load the module but php has given me this error: Warning: Invalid library (maybe not a PHP library) 'acme.so' in /u/WEB/www.testtest.it/denis/index.php on line 4 Why does it give me this error? Thank you in advance for your help, Denis