I assumed the mistake in my WSDL file, since I knew the function “foo” existed.

If you get the “Procedure ‘foo’ not present”-error, too you either:

• forgot to add the procedure “foo” to your SOAP server implementation, or
• you are using a cached version of your WSDL file at the server side.

I discovered that sometimes a cached WSDL file is used, although you are using this code:
ini_set("soap.wsdl_cache_enabled", "0");
If you aren’t sure whether you are using a cached version or not, you can either

• reboot the server to clean the cache, or
• delete all files in /tmp/ starting with “wsdl-” (rm /tmp/wsdl-*)

If not a cached WSDL file is causing the problem, you should check if you really have a function “foo” in your implementation.