When I create a new account on WHM, the new account has a default "public_html" directory which permission is 750.

The permission doesn't allow my users to access php files and server returned "500 Internal Server Error".

I have to manually change the permission of "public_html" to 755 and php scripts is working after that.

My php handler for Apache is suphp, what can I do to solve the 500 error without manually chmod "public_html"?

if your suphp installation was working correctly, i believe it should not require a chmod to allow global read/execute.

Also, check to make sure that the user:group in each virtualhost is the same as the user:group of the files contained in each (sub)domain folder/site